In this work the analysis of the dependence of the porous InP morphology on the type of reacting anion is presented. It is shown that nanoporous InP layers are obtained under certain conditions of electrochemical etching.
При цитуванні документа, використовуйте посилання http://essuir.sumdu.edu.ua/handle/123456789/9352У даній роботі подано аналіз залежності морфології поруватого фосфіду індію від типу аніону що бере участь у реакції пороутворення. Показано, що за певних умов електролітичного травлення, можливе отримання нанопоруватих шарів InP.
При цитировании документа, используйте ссылку http://essuir.sumdu.edu.ua/handle/123456789/9352В данной работе представлен анализ зависимости морфологии пористого фосфида индия от типа участвующего в реакции аниона. Показано, что при определенных условиях электролитического травления, получаются нанопористые слои InP.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/935

Kidalov, V.V

Suchikova, Ya.A.

Sukach, G.A.

SocioEconomic Challenges Journal (Sumy State University)

 
 
 
J. Nano- Electron. Phys. 
1 (2009) No4, P. 111-118 
2009 SumDU 
(Sumy State University) 
 
 
111 
PACS numbers: 61.43.Gt, 78.30.Fs, 78.55.m 
 
EFFECT OF THE TYPE OF ELECTROLYTE ÀNION ON THE POROUS InP 
MORPHOLOGY OBTAINED BY THE ELECTROCHEMICAL ETCHING 
 
Ya.A. Suchikova1, V.V. Kidalov1, G.A. Sukach2 
 
1 Berdyansk State Pedagogical University, 
 4, Shmidta Str., 71100, Berdyansk, Ukraine 
 E-mail: V.V.Kidalov@mail.ru 
2 V. Lashkaryov`s Institute of Semiconductor Physics NASU, 
 41, Pr. Nauki, 03028, Kiev, Ukraine 
 
In this work the analysis of the dependence of the porous InP morphology on the type 
of  reacting anion is  presented.  It  is  shown that nanoporous InP layers  are obtained 
under certain conditions of electrochemical etching. 
 
Keywords: POROUS InP, ELECTROCHEMICAL ETCHING, SCANNING ELECTRON 
MICROSCOPY, NANOSTRUCTURES, THRESHOLD VOLTAGE. 
 
(Received 05 November 2009, in final form 01 December 2009) 
 
1. INTRODUCTION 
 
At present, various methods for the self-organized formation of nanostruc-
tures are actively developing due to their use for the electronic and photon 
devices, chemical and biochemical sensors [1, 2], etc. In À3Â5 semiconduc-
tors for nanostructure formation a rather expensive method of molecular-
beam epitaxy or metal-organic vapor-phase epitaxy (MOVPE) is applied. The 
possible alternative to these methods is the use of electrochemical processes 
by  analogy  with  the  porous  silicon  production  [3].  Feature  of  the  electro-
chemical process is the low-temperature process, the slight surface damage, 
the  process  simplicity  and  its  low  cost.  Electrochemical  methods  allow  to  
obtain the high pore density, which is unattainable if use other methods. 
 To obtain the porous InP structures, HCl, HBr, HF and KOH electrolytes 
[4, 5] are used. Established, that the structures formed in HF solutions 
demonstrate visible photoluminescence (PhL) in the spectral range from 
yellow color to red color, while in the samples finished in HCl and HBr 
electrolytes the essential PhL in the visible range was not observed. Illumi-
nation [6] together with the electrolyte type has significant influence on the 
pore morphology. At the high illumination level instead of the pore formation 
the electrolytic polishing takes place. The most qualitative structures were 
obtained in the darkness. 
 The study of the properties of p-InP (100) porous structures obtained by 
the electrochemical etching in HBr and HF solutions [7] showed that their 
morphology and optical properties strongly depend on the halogen in electro-
lyte and the etching conditions. The structure formed in HF solution was 
characterized by the presence of two PhL bands: the first band was in the 
range of 630-700 nm (most probably it was connected with the dimensional 
quantification effects), the second band was in the range of 530-590 nm (its 
 
 
 
112 Y.A. SUCHIKOVA, V.V. KIDALOV, G.A. SUKACH  
 
nature was in the presence of oxides on the porous material surface). And 
the spectrums of PhL samples finished in HBr were characterized by the 
presence of only one band (520 nm), which is also explained by the presence 
of oxides. 
 The same dependence on the electrochemical conditions and the type of 
halogen (Cl, F) in electrolyte was established in [8] for porous structures 
grown  by  the  electrochemical  etching  on  n-InP (100) in the darkness. The 
significant influence of the surface state on the pore nucleation was also 
detected. It was shown that the pore formation potential is lower for the 
surfaces with defects than for the undamaged ones. 
 Results of the integrated study of the porous InP structures obtained by 
the electrochemical etching (HCl, HF and HBr) and published in [9] showed 
that the features of the PhL spectrums of porous InP are connected with the 
deformation of the interatomic spacings in quasi-molecules of p-InP surface 
layer and with the change of the electronic structure of clusters in porous 
InP layers. 
 
2. STATEMENT OF THE PROBLEM 
 
Although the investigation results of the surface morphology and the optical 
properties of porous InP obtained by the electrochemical etching in different 
electrolytes are presented by a number of scientific groups, there is a 
limited amount of publications devoted to the analysis of the dependence of 
the  porous  InP  structure  on  the  etching  regimes.  The  present  paper  is  
dedicated  to  the  study  of  the  effect  of  the  anion  type  on  the  porous  InP  
properties that is relevant from the point of view of the control of the pore 
diameter and the porous surface quality. 
 
3. EXPERIMENTAL TECHNIQUE 
 
Indium phosphide monocrystals were produced in the research laboratory of 
the “Molecular Technology GmbH” Company (Berlin). Thickness of the samples 
was 1 mm. Plates were cut out perpendicular to the growth axis and polished 
on both sides. 
 InP samples with different surface orientation of the n- and p-type and 
with different charge carrier concentrations were chosen for the experiment. 
 Electrochemical etching was performed using the standard plant in the 
electrolytic cell with the platinum cathode. The plant scheme is presented in 
Fig. 1. 
 Solutions of hydrofluoric and hydrochloric acids with different concen-
trations were taken as the electrolytes. Etchants based on HF and HCl with 
added iodides (KOH), ethanol and nitric acid were also used. The experiment 
was carried out at room temperature. 
 Samples were thoroughly purified before the experiment. There were the 
following purification steps: 
1. degreasing in hot (75-80 qC) peroxide-ammonia solution; 
2. treatment in hot (90-100 qC) concentrated nitric acid (metal ion removal); 
3. washing in distilled water; 
4. drying of the plates using centrifuge in the purified dry air. 
 After experiment the samples were purified in acetone and isopropanol, 
washed in distilled water and dried in extra pure hydrogen, whereupon they 
were undergone natural aging during three days. 
 
 
 
 EFFECT OF THE TYPE OF ELECTROLYTE ÀNION ON THE POROUS … 113 
 
 Morphology of the obtained porous structures was investigated using the 
scanning electron microscope JSM-6490. Chemical composition was studied 
with the EDAX method, and the diffractometric investigations were carried 
out using the diffractometer DRON-3M. 
 
 
 
Fig. 1 – Plant for the semiconductor etching 
 
4. RESULTS AND DISCUSSION 
 
Morphology analysis of the tested samples obtained using the scanning 
electron microscopy (SEM) showed that almost in all cases the active pore-
formation was observed. In the conditions when pore-formation is the domi-
nant electrochemical process, which takes place at the given value of the 
polarizing voltage on the monocrystal semiconductor anode, the steady-state 
configuration of the porous layer surface is formed by the time of the maxi-
mum current density attainment. 
 Under equal conditions (identical crystals, equal charge and anion concen-
tration in the solution) the electrolytic reaction rate depends on the type of 
reacting anion. Depending on the degree of dissociation into ions electrolytes 
can be classified as strong and weak ones. Their behavior on dissolving is 
different. Some molecules of weak electrolytes dissociate into ions under sol-
vent action. Their dissociation is the reversible process since ions are easily 
associated in collisions, therefore in solutions of weak electrolytes the dyna-
mic equilibrium between ions and undissociated molecules is attained. When 
strong electrolytes dissolve the dissociation occurs almost completely, ionic 
crystals or molecules dissociate with the formation of aquated (solvated) ions. 
Some of the most important acids, namely, HNO3, H2SO4, HClO4, HCl, HBr 
belong to strong electrolytes. Among weak electrolytes are most of inor-
ganic compounds, such as H2CO3, H2S, HCN, HF. 
 
4.1 HF-based etchants 
 
Small size of F atom substantially influences the fluoride properties. F atom 
in chemical compounds practically always is negatively charged. F is the 
most active oxidant among the elementary substances, it reacts with almost 
all substances. The feature of fluorine chemistry is the presence of stable 
hydrogen bonds H–F. The average degree of association of HF molecules in a 
liquid state is closed to 6. Etching acid is considered to be the half-strength 
acid (the dissociation constant is 6,810–4). Electronegativity of F– atoms in 
the series F–, Cl–, Br– is the largest and is equal to 4 on the Pauling scale.
THREE-ELECTRODE 
POTENTIOSTATE 
 
 
 
114 Y.A. SUCHIKOVA, V.V. KIDALOV, G.A. SUKACH  
 
 In the halogenide-ions series the minimum voltage value of the formation 
initiation always corresponds to fluorine anion. Morphology of the porous 
samples obtained using etching acid demonstrates the net of meso- and macro-
pores. Formation of such pores is often connected with the displacement of 
defects and dislocations on the crystal surface. In this case the substantial 
overetching of the surface is observed (Fig. 2). 
 
 
 
Fig. 2 – Morphology of porous n-InP (111); electrolyte HF : H2O   1 : 1, j   80 mA/cm2, 
t   10 min 
 
 In Fig. 2 it is possible to see clearly the etched regions that imply about 
very “harsh” etching conditions. Porous surface demonstrates the developed 
morphology with the formed large etching pits. Such surface has a huge 
effective area in comparison with the monocrystal one, but it is imperfect 
enough for use as the substrate for heterostructure production. In this case, 
in order to decrease the electrolyte effect on the porous structure formation 
it is reasonable to change the etching regimes (time, current density) for the 
more soft ones or use more dilute etchant solution. 
 When adding ethanol to this solution the value of threshold voltage of the 
pore-formation initiation substantially increases, and here porous layer has more 
qualitative structure that is appeared in the decrease in the pore size (Fig. 3). 
 
 
 
Fig. 3 – Morphology of porous n-InP (111); electrolyte HF : H2O : C2H5OH   1 : 1 : 2, 
j   40 mA/cm2, t   15 min 
 
 
 
 EFFECT OF THE TYPE OF ELECTROLYTE ÀNION ON THE POROUS … 115 
 
 In general, ethyl alcohol is an organic diluent of electrolyte. When adding 
it to the aqueous solution of etching acid (the component ratio is HF : H2O : 
C2H5OH   1 : 1 : 2) the electrolytic reaction rate slows down, and therefore 
more time is needed to obtain the porous structures. 
 The authors of [11] note that ethyl alcohol is used for the improvement of 
HF penetration into pores. In our case this leads to the formation of the dense 
net of anisotropically propagating pores along the directions 111A and 111B 
(Fig. 4). Degree of porosity is about 30% of the total area of the sample. 
 
 
 
Fig. 4 – Anisotropic pore propagation along the directions +111, A and +111, B 
 
 In Fig. 5 we present the cleavage of porous n-InP (111) obtained in the 
HF-based etchant. Irritated area, the appearance of which can be caused by 
the dislocation displacement, is clearly observed under the crystal surface. 
 
 
 
Fig. 5 – Cleavage of porous n-InP (111), electrolyte HF : H2O : C2H5OH   1 : 1 : 2, 
j   50 mA/cm2, t   10 min 
 
 The following fact is found to be interesting: the crystal surface is not so 
irritated as an area under it. This implies about the pore-formation inside 
the crystal as well, where pores can be combined due to the thinning of the 
walls and their accumulation around defects. 
 
 
 
116 Y.A. SUCHIKOVA, V.V. KIDALOV, G.A. SUKACH  
 
4.2 HCl-based etchants 
 
Hydrochloric acid is the aqueous solution of hydrogen chloride, it is strong 
(monoatomic) acid. The maximum concentration of such solution at 200qC is 
38 at% and the density is 1,19 g/cm3. HCl is most often used during the 
electrochemical etching of the crystals that is explained by the possibility of 
this acid to easily dissociate into ions. 
 HCl-based etchants allow to obtain the layer composed of the nanopores 
mainly. Fig. 6 demonstrates the ordered assembly of pores formed on the mono-
crystal InP substrate during the etching in 5% hydrochloric acid solution. 
Pores appeared all over the sample surface. The average pore size is 40 nm 
that implies about the nanodimensionality of the given structure. The wall 
size between pores is in the range 5-10 nm. Such a result is important techno-
logically, since the quality of porous films is determined by the nanostruc-
ture sizes, degree of porosity and uniformity of the pore distribution over 
the sample surface. The smaller pore size and the larger porosity percent, 
the more qualitative porous structure is. Degree of porosity is about 60% of 
the total area of the sample. 
 
 
 
Fig. 6 – Morphology of porous n-InP (100) obtained by the electrochemical etching in 
5% HCl solution, t   5 min 
 
 In Fig. 7 we present the cleavage of the porous sample obtained in 5% hydro-
chloric acid solution. This figure demonstrates the long parallel pore channels 
placed strictly perpendicular to the crystal surface. Such pores are directed 
along the lines of flow and reach the depth of 60 Pm inside the sample. 
 
4.3 Salts and acids addition to the electrolyte solution 
 
Iodide addition to the fluoride solution essentially influences the pore-for-
mation process. When adding KI (potassium iodide) to 50% etching acid 
solution, the pore-formation process slows down somewhat, what the current-
voltage characteristics taken during anodization imply about. Morphology of 
the samples obtained in such solution demonstrates the decrease of the pore 
inlets (degree of porosity is about 15%). However, the branching of the pore 
channels under the crystal surface increases. Here the irregular layer preceding 
the long mutually parallel pores widens up to 2 Pm (in comparison with the 
irregular pore layer formed in the aqueous-fluoride solution, which in this 
case is 0,5-1 Pm). 
 
 
 
 EFFECT OF THE TYPE OF ELECTROLYTE ÀNION ON THE POROUS … 117 
 
 
 
Fig. 7 – Cleavage of porous n-InP (100) obtained by the electrochemical etching in 
5% HCl solution, t   5 min 
 
 Nitric acid addition to the chloride solutions accelerates the electrolytic 
reaction rate and allows to obtain the porous surfaces with the more densely 
packed pores (degree of porosity is 60-70%). However, as in the case of iodides, 
nitric acid influences the thickness of irregular porous layer. In this case the 
given value is about 1,8 Pm (in the absence of nitric acid it is 0,4-0,7 Pm). 
 
5. CONCLUSIONS 
 
In this paper we present the analysis of the dependence of the porous InP 
morphology on the type of reacting anion. The main conclusions of the work 
are reproduced below. 
 Under equal conditions (identical crystals, equal charge and anion concen-
tration in the solution) the electrolytic reaction rate depends on the type of 
reacting anion. 
 In the halogenide-ions series the minimum voltage value of the formation 
initiation always corresponds to fluorine anion. Morphology of the porous 
samples obtained using etching acid demonstrates the net of meso- and macro-
pores. Formation of such pores is often connected with the displacement of 
defects and dislocations on the crystal surface. 
 HCl-based etchants allow to obtain the layer composed of the nanopores 
mainly. And therefore this etchant can be considered as the most appropriate 
for the porous InP nanostructure production. 
 When adding KI (potassium iodide) to 50% etching acid solution, the 
pore-formation process slows down. 
 Morphology of the samples obtained in such solution demonstrates the 
decrease of the pore inlets (degree of porosity is about 15%). However, the 
branching of the pore channels under the crystal surface increases. 
 Nitric acid addition to the chloride solutions accelerates the electrolytic 
reaction rate and allows to obtain the porous surfaces with the more densely 
packed pores (degree of porosity is 60-70%). 
 The  factors  stated  above  enable  to  produce  the  porous  structures  with  
specified parameters that corresponds to the modern electronics and techniques 
requirements. 
 
 
 
 
 
118 Y.A. SUCHIKOVA, V.V. KIDALOV, G.A. SUKACH  
 
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Effect of the type of electrolyte аnion on the porous InP morphology obtained by the electrochemical etching

Electronic Sumy State University Institutional Repository

   J. Nano- Electron. Phys. 1 (2009) No4, P. 111-118 2009 SumDU (Sumy State University)   111 PACS numbers: 61.43.Gt, 78.30.Fs, 78.55.m  EFFECT OF THE TYPE OF ELECTROLYTE ÀNION ON THE POROUS InP MORPHOLOGY OBTAINED BY THE ELECTROCHEMICAL ETCHING  Ya.A. Suchikova1, V.V. Kidalov1, G.A. Sukach2  1 Berdyansk State Pedagogical University,  4, Shmidta Str., 71100, Berdyansk, Ukraine  E-mail: V.V.Kidalov@mail.ru 2 V. Lashkaryov`s Institute of Semiconductor Physics NASU,  41, Pr. Nauki, 03028, Kiev, Ukraine  In this work the analysis of the dependence of the porous InP morphology on the type of  reacting anion is  presented.  It  is  shown that nanoporous InP layers  are obtained under certain conditions of electrochemical etching.  Keywords: POROUS InP, ELECTROCHEMICAL ETCHING, SCANNING ELECTRON MICROSCOPY, NANOSTRUCTURES, THRESHOLD VOLTAGE.  (Received 05 November 2009, in final form 01 December 2009)  1. INTRODUCTION  At present, various methods for the self-organized formation of nanostruc-tures are actively developing due to their use for the electronic and photon devices, chemical and biochemical sensors [1, 2], etc. In À3Â5 semiconduc-tors for nanostructure formation a rather expensive method of molecular-beam epitaxy or metal-organic vapor-phase epitaxy (MOVPE) is applied. The possible alternative to these methods is the use of electrochemical processes by  analogy  with  the  porous  silicon  production  [3].  Feature  of  the  electro-chemical process is the low-temperature process, the slight surface damage, the  process  simplicity  and  its  low  cost.  Electrochemical  methods  allow  to  obtain the high pore density, which is unattainable if use other methods.  To obtain the porous InP structures, HCl, HBr, HF and KOH electrolytes [4, 5] are used. Established, that the structures formed in HF solutions demonstrate visible photoluminescence (PhL) in the spectral range from yellow color to red color, while in the samples finished in HCl and HBr electrolytes the essential PhL in the visible range was not observed. Illumi-nation [6] together with the electrolyte type has significant influence on the pore morphology. At the high illumination level instead of the pore formation the electrolytic polishing takes place. The most qualitative structures were obtained in the darkness.  The study of the properties of p-InP (100) porous structures obtained by the electrochemical etching in HBr and HF solutions [7] showed that their morphology and optical properties strongly depend on the halogen in electro-lyte and the etching conditions. The structure formed in HF solution was characterized by the presence of two PhL bands: the first band was in the range of 630-700 nm (most probably it was connected with the dimensional quantification effects), the second band was in the range of 530-590 nm (its    112 Y.A. SUCHIKOVA, V.V. KIDALOV, G.A. SUKACH   nature was in the presence of oxides on the porous material surface). And the spectrums of PhL samples finished in HBr were characterized by the presence of only one band (520 nm), which is also explained by the presence of oxides.  The same dependence on the electrochemical conditions and the type of halogen (Cl, F) in electrolyte was established in [8] for porous structures grown  by  the  electrochemical  etching  on  n-InP (100) in the darkness. The significant influence of the surface state on the pore nucleation was also detected. It was shown that the pore formation potential is lower for the surfaces with defects than for the undamaged ones.  Results of the integrated study of the porous InP structures obtained by the electrochemical etching (HCl, HF and HBr) and published in [9] showed that the features of the PhL spectrums of porous InP are connected with the deformation of the interatomic spacings in quasi-molecules of p-InP surface layer and with the change of the electronic structure of clusters in porous InP layers.  2. STATEMENT OF THE PROBLEM  Although the investigation results of the surface morphology and the optical properties of porous InP obtained by the electrochemical etching in different electrolytes are presented by a number of scientific groups, there is a limited amount of publications devoted to the analysis of the dependence of the  porous  InP  structure  on  the  etching  regimes.  The  present  paper  is  dedicated  to  the  study  of  the  effect  of  the  anion  type  on  the  porous  InP  properties that is relevant from the point of view of the control of the pore diameter and the porous surface quality.  3. EXPERIMENTAL TECHNIQUE  Indium phosphide monocrystals were produced in the research laboratory of the “Molecular Technology GmbH” Company (Berlin). Thickness of the samples was 1 mm. Plates were cut out perpendicular to the growth axis and polished on both sides.  InP samples with different surface orientation of the n- and p-type and with different charge carrier concentrations were chosen for the experiment.  Electrochemical etching was performed using the standard plant in the electrolytic cell with the platinum cathode. The plant scheme is presented in Fig. 1.  Solutions of hydrofluoric and hydrochloric acids with different concen-trations were taken as the electrolytes. Etchants based on HF and HCl with added iodides (KOH), ethanol and nitric acid were also used. The experiment was carried out at room temperature.  Samples were thoroughly purified before the experiment. There were the following purification steps: 1. degreasing in hot (75-80 qC) peroxide-ammonia solution; 2. treatment in hot (90-100 qC) concentrated nitric acid (metal ion removal); 3. washing in distilled water; 4. drying of the plates using centrifuge in the purified dry air.  After experiment the samples were purified in acetone and isopropanol, washed in distilled water and dried in extra pure hydrogen, whereupon they were undergone natural aging during three days.     EFFECT OF THE TYPE OF ELECTROLYTE ÀNION ON THE POROUS … 113   Morphology of the obtained porous structures was investigated using the scanning electron microscope JSM-6490. Chemical composition was studied with the EDAX method, and the diffractometric investigations were carried out using the diffractometer DRON-3M.    Fig. 1 – Plant for the semiconductor etching  4. RESULTS AND DISCUSSION  Morphology analysis of the tested samples obtained using the scanning electron microscopy (SEM) showed that almost in all cases the active pore-formation was observed. In the conditions when pore-formation is the domi-nant electrochemical process, which takes place at the given value of the polarizing voltage on the monocrystal semiconductor anode, the steady-state configuration of the porous layer surface is formed by the time of the maxi-mum current density attainment.  Under equal conditions (identical crystals, equal charge and anion concen-tration in the solution) the electrolytic reaction rate depends on the type of reacting anion. Depending on the degree of dissociation into ions electrolytes can be classified as strong and weak ones. Their behavior on dissolving is different. Some molecules of weak electrolytes dissociate into ions under sol-vent action. Their dissociation is the reversible process since ions are easily associated in collisions, therefore in solutions of weak electrolytes the dyna-mic equilibrium between ions and undissociated molecules is attained. When strong electrolytes dissolve the dissociation occurs almost completely, ionic crystals or molecules dissociate with the formation of aquated (solvated) ions. Some of the most important acids, namely, HNO3, H2SO4, HClO4, HCl, HBr belong to strong electrolytes. Among weak electrolytes are most of inor-ganic compounds, such as H2CO3, H2S, HCN, HF.  4.1 HF-based etchants  Small size of F atom substantially influences the fluoride properties. F atom in chemical compounds practically always is negatively charged. F is the most active oxidant among the elementary substances, it reacts with almost all substances. The feature of fluorine chemistry is the presence of stable hydrogen bonds H–F. The average degree of association of HF molecules in a liquid state is closed to 6. Etching acid is considered to be the half-strength acid (the dissociation constant is 6,810–4). Electronegativity of F– atoms in the series F–, Cl–, Br– is the largest and is equal to 4 on the Pauling scale.THREE-ELECTRODE POTENTIOSTATE    114 Y.A. SUCHIKOVA, V.V. KIDALOV, G.A. SUKACH    In the halogenide-ions series the minimum voltage value of the formation initiation always corresponds to fluorine anion. Morphology of the porous samples obtained using etching acid demonstrates the net of meso- and macro-pores. Formation of such pores is often connected with the displacement of defects and dislocations on the crystal surface. In this case the substantial overetching of the surface is observed (Fig. 2).    Fig. 2 – Morphology of porous n-InP (111); electrolyte HF : H2O   1 : 1, j   80 mA/cm2, t   10 min   In Fig. 2 it is possible to see clearly the etched regions that imply about very “harsh” etching conditions. Porous surface demonstrates the developed morphology with the formed large etching pits. Such surface has a huge effective area in comparison with the monocrystal one, but it is imperfect enough for use as the substrate for heterostructure production. In this case, in order to decrease the electrolyte effect on the porous structure formation it is reasonable to change the etching regimes (time, current density) for the more soft ones or use more dilute etchant solution.  When adding ethanol to this solution the value of threshold voltage of the pore-formation initiation substantially increases, and here porous layer has more qualitative structure that is appeared in the decrease in the pore size (Fig. 3).    Fig. 3 – Morphology of porous n-InP (111); electrolyte HF : H2O : C2H5OH   1 : 1 : 2, j   40 mA/cm2, t   15 min     EFFECT OF THE TYPE OF ELECTROLYTE ÀNION ON THE POROUS … 115   In general, ethyl alcohol is an organic diluent of electrolyte. When adding it to the aqueous solution of etching acid (the component ratio is HF : H2O : C2H5OH   1 : 1 : 2) the electrolytic reaction rate slows down, and therefore more time is needed to obtain the porous structures.  The authors of [11] note that ethyl alcohol is used for the improvement of HF penetration into pores. In our case this leads to the formation of the dense net of anisotropically propagating pores along the directions 111A and 111B (Fig. 4). Degree of porosity is about 30% of the total area of the sample.    Fig. 4 – Anisotropic pore propagation along the directions +111, A and +111, B   In Fig. 5 we present the cleavage of porous n-InP (111) obtained in the HF-based etchant. Irritated area, the appearance of which can be caused by the dislocation displacement, is clearly observed under the crystal surface.    Fig. 5 – Cleavage of porous n-InP (111), electrolyte HF : H2O : C2H5OH   1 : 1 : 2, j   50 mA/cm2, t   10 min   The following fact is found to be interesting: the crystal surface is not so irritated as an area under it. This implies about the pore-formation inside the crystal as well, where pores can be combined due to the thinning of the walls and their accumulation around defects.    116 Y.A. SUCHIKOVA, V.V. KIDALOV, G.A. SUKACH   4.2 HCl-based etchants  Hydrochloric acid is the aqueous solution of hydrogen chloride, it is strong (monoatomic) acid. The maximum concentration of such solution at 200qC is 38 at% and the density is 1,19 g/cm3. HCl is most often used during the electrochemical etching of the crystals that is explained by the possibility of this acid to easily dissociate into ions.  HCl-based etchants allow to obtain the layer composed of the nanopores mainly. Fig. 6 demonstrates the ordered assembly of pores formed on the mono-crystal InP substrate during the etching in 5% hydrochloric acid solution. Pores appeared all over the sample surface. The average pore size is 40 nm that implies about the nanodimensionality of the given structure. The wall size between pores is in the range 5-10 nm. Such a result is important techno-logically, since the quality of porous films is determined by the nanostruc-ture sizes, degree of porosity and uniformity of the pore distribution over the sample surface. The smaller pore size and the larger porosity percent, the more qualitative porous structure is. Degree of porosity is about 60% of the total area of the sample.    Fig. 6 – Morphology of porous n-InP (100) obtained by the electrochemical etching in 5% HCl solution, t   5 min   In Fig. 7 we present the cleavage of the porous sample obtained in 5% hydro-chloric acid solution. This figure demonstrates the long parallel pore channels placed strictly perpendicular to the crystal surface. Such pores are directed along the lines of flow and reach the depth of 60 Pm inside the sample.  4.3 Salts and acids addition to the electrolyte solution  Iodide addition to the fluoride solution essentially influences the pore-for-mation process. When adding KI (potassium iodide) to 50% etching acid solution, the pore-formation process slows down somewhat, what the current-voltage characteristics taken during anodization imply about. Morphology of the samples obtained in such solution demonstrates the decrease of the pore inlets (degree of porosity is about 15%). However, the branching of the pore channels under the crystal surface increases. Here the irregular layer preceding the long mutually parallel pores widens up to 2 Pm (in comparison with the irregular pore layer formed in the aqueous-fluoride solution, which in this case is 0,5-1 Pm).     EFFECT OF THE TYPE OF ELECTROLYTE ÀNION ON THE POROUS … 117    Fig. 7 – Cleavage of porous n-InP (100) obtained by the electrochemical etching in 5% HCl solution, t   5 min   Nitric acid addition to the chloride solutions accelerates the electrolytic reaction rate and allows to obtain the porous surfaces with the more densely packed pores (degree of porosity is 60-70%). However, as in the case of iodides, nitric acid influences the thickness of irregular porous layer. In this case the given value is about 1,8 Pm (in the absence of nitric acid it is 0,4-0,7 Pm).  5. CONCLUSIONS  In this paper we present the analysis of the dependence of the porous InP morphology on the type of reacting anion. The main conclusions of the work are reproduced below.  Under equal conditions (identical crystals, equal charge and anion concen-tration in the solution) the electrolytic reaction rate depends on the type of reacting anion.  In the halogenide-ions series the minimum voltage value of the formation initiation always corresponds to fluorine anion. Morphology of the porous samples obtained using etching acid demonstrates the net of meso- and macro-pores. Formation of such pores is often connected with the displacement of defects and dislocations on the crystal surface.  HCl-based etchants allow to obtain the layer composed of the nanopores mainly. And therefore this etchant can be considered as the most appropriate for the porous InP nanostructure production.  When adding KI (potassium iodide) to 50% etching acid solution, the pore-formation process slows down.  Morphology of the samples obtained in such solution demonstrates the decrease of the pore inlets (degree of porosity is about 15%). 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Effect of the type of electrolyte аnion on the porous InP morphology obtained by the electrochemical etching

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