We have investigated the electrical properties of Pd/Ru Schottky contacts on n-GaN as a function of annealing temperature by current-voltage (I-V) and capacitance-voltage (C-V) measurements. The Schottky barrier height of the as-deposited Pd/Ru contact is found to be 0.67 eV (I-V) and 0.79 eV (C-V), respectively. Measurements showed that the Schottky barrier height increased from 0.68 eV (I-V) and 0.80 eV (C-V) to 0.80 eV (I-V) and 0.96 eV (C-V) as the annealing temperature is varied from 200 °C to 300 °C. Upon annealing at 400 °C and 500 °C, the Schottky barrier height decreased to 0.73 eV (I-V) and 0.85 eV (C-V) and 0.72 eV (I-V) and 0.84 eV (C-V), respectively. It is noted that the barrier height further decreased to 0.59 eV (I-V) and 0.72 eV (C-V) when the contact is annealed at 600 °C. The change of Schottky barrier heights and ideality factors with annealing temperature may be due to the formation of interfacial compounds at the Ru/Pd/n-GaN interface.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/2788

Nanda, Kumar Reddy N.

Rajagopal, Reddy V.

English

Electronic Sumy State University Institutional Repository

   J. Nano- Electron. Phys. 3 (2011) No1, P. 921-925  2011 SumDU (Sumy State University)   921 PACS numbers: 85.30.Hi, 85.30.Kk  THERMAL ANNEALING BEHAVIOUR ON ELECTRICAL PROPERTIES OF Pd/Ru SCHOTTKY CONTACTS ON n-TYPE GaN  N. Nanda Kumar Reddy, V. Rajagopal Reddy     Sri Venkateswara University,    Department of Physics, Semiconductor Devices Lab, Tirupati-517502, India   E-mail: nandasvu@gmail.com  We have investigated the electrical properties of Pd/Ru Schottky contacts on n-GaN as  a  function  of  annealing  temperature  by  current-voltage  (I-V)  and  capacitance-voltage (C-V) measurements. The Schottky barrier height of the as-deposited Pd/Ru contact is found to be 0.67 eV (I-V) and 0.79 eV (C-V), respectively. Measurements showed that the Schottky barrier height increased from 0.68 eV (I-V) and 0.80 eV (C-V) to 0.80 eV (I-V) and 0.96 eV (C-V) as the annealing temperature is varied from 200  C to  300  C.  Upon  annealing  at  400  C  and  500  C,  the  Schottky  barrier  height decreased  to  0.73 eV  (I-V)  and  0.85 eV  (C-V)  and  0.72 eV  (I-V)  and  0.84 eV  (C-V), respectively. It is noted that the barrier height further decreased to 0.59 eV (I-V) and 0.72 eV (C-V) when the contact is annealed at 600  C. The change of Schottky barrier heights and ideality factors with annealing temperature may be due to the formation of interfacial compounds at the Ru/Pd/n-GaN interface.  Keywords: SCHOTTKY BARRIER DIODE, THERMAL ANNEALING TEMPERATURE, I-V AND C-V CHARACTERISTICS, LEAKAGE CURRENT, BARRIER HEIGHT.  (Received 04 February 2011, in final form 30 November 2011)  1.  INTRODUCTION  Because  of  its  wide  band  gap,  high  electron  saturation  velocity,  large breakdown field and thermal stability, GaN is an attractive candidate for high temperature/high-power electronics [1]. Devices such as metal-semiconductor field effect transistors (MESFETs), high-electron-mobility transistors (HEMTs) and hetrojunction bipolar transistors (HBTs) have been fabricated on GaN [1]. In order to enhance the efficiency of these devices, thermally stable ohmic and Schottky  contacts  are  required.  A  number  of  metals  have  been  used  as Schottky contacts to GaN by several research groups eg Pt, Au, Ni, Re and Ir. In the present work, we have fabricated and investigated thermal annealing temperature  effects  on  the  electrical  characteristics  of  Pd/Ru  Schottky contacts to n-type GaN.  2.  EXPERIMENTAL DETAILS  In  the  present  work,  2  m  thick  Si  doped  GaN  films  are  used,  which  are grown by metalorganic chemical vapour deposition (MOCVD) on c-plane Al2O3 substrates.  The  electron  concentration  obtained  by  means  of  Hall  measure-ments is   5.07   10 17 cm 3. The n-GaN layer is first ultrasonically degreased with  warm  trichloroethylene  followed  by  acetone  and  methanol  for  5 min each.  This  degreased  layer  is  then  dipped  into  boiling  aqua  regia    922  N. NANDA KUMAR REDDY, V. RAJAGOPAL REDDY   [HNO3 : HCL   1 : 3] for 10 min to remove the surface oxides and rinsed in deionized  (DI)  water.  Ohmic  contact  is  formed  by  depositing  Ti (20 nm)/ Al (100 nm) on a portion of the sample by electron beam evaporation system. Rapid  thermal  annealing  is  conducted  at  650  C  for  3 min  in  nitrogen ambient.  Subsequently,  a  Schottky  metallization  scheme  is  achieved  by depositing  Pd (20 nm)/Ru (30 nm)  bilayer  with  a  diameter  of  0.7 mm through a stainless steel mask in an electron beam evaporation system at a vacuum pressure of 7   10 6 mbar. In order to study the thermal annealing effects  on  Pd/Ru  Schottky  contacts  to  n-GaN,  samples  were  annealed  at 200  C, 300  C, 400  C, 500  C and 600  C for 1 min in N2 atmosphere in a rapid  thermal  annealing  (RTA)  system.  Current-voltage  (I-V)  and capacitance  (C-V)  characteristics  of  the  Schottky  diode  were  measured  at room temperature using Keithley source measuring unit (model No. 2400) and automated DLS - 83D spectrometer (SEMILAB, Hungary), respectively.   3.  RESULTS AND DISCUSSION  We  analyze  the  experimental  I-V  characteristics  by  the  forward  bias thermionic emission (TE) theory given as follows [2],    0 exp( )[1 exp( )]qV qVIInkT kT  (1)  Where   bo qI AA TkT*20 exp( )   (2)  here  I0  is  the  saturation  current,  q  is  the  electronic  charge,  A*  is  the effective Richardson’s constant and equals to 26.4 A/cm2K2 for n-GaN [3], A  is  the  effective  diode  area,  k  is  the  Boltzmann  constant,  bo  is  the Schottky barrier height and n is the ideality factor.     Fig. 1 – I-V characteristics  of  Pd/Ru Schottky contact on  n-GaN as  a function  of annealing temperatures  The ideality factor is calculated from the slope of the linear region of the forward-bias lnI versus V plots and can be written from equation (1) as,        THERMAL ANNEALING BEHAVIOUR ON ELECTRICAL…  923   ()(ln )q dVnkT d I  (3)  The Schottky barrier height ( bo) is determined from the extrapolated I0 and is given by,   *20ln( ) bokT AA TqI  (4)    Using linear curve fitting for the forward characteristics of lnI versus V data, the barrier height ( bo) and ideality factor (n) can be determined from the y-intercept and the slope of the fitted curves respectively.   From Fig. 1, the reverse leakage current value at – 2 V for the as-deposited contact is measured to be 3.79   10  6 A. For the samples annealed at 200  C, 300  C,  400  C,  500  C  and  600  C  the  reverse  leakage  current  values  are 9.32   10  6, 3.76   10  9, 5.60   10  7, 9.29   10  7 and 4.02   10  4 A, respec-tively. The smallest value of leakage current 3.76   10  9 A is observed for the  contact  annealed  at  300  C.  Calculations  yielded  a  Schottky  barrier height of 0.67 eV for the as-deposited Pd/Ru contact. The extracted barrier heights  of  the  annealed  Pd/Ru  Schottky  contacts  are  0.68 eV  for  200  C, 0.80 eV for 300  C, 0.73 eV for 400  C, 0.72 eV for 500  C and 0.59 eV for 600  C.  The  ideality  factor  of  the  as-deposited  Ru/Pd/n-GaN  Schottky contact is found to be 1.62. However, the ideality factor increased to 2.04 upon annealing at 600  C. Ideality factors above unity has been attributed to interface  states  due  to  thin  oxide  layer  between  the  metal  and  the semiconductor, including other contaminants, tunneling currents in highly doped  semiconductors,  image-force  lowering  of  the  Schottky  barrier  in electric field at the interface and generation-recombination currents with in the depletion region [2].     Fig. 2 – Plot of 1/C2 versus V for the Pd/Ru Schottky contacts annealed at different temperatures  A  plot  of  1/C2  as  a  function  of  bias  voltage  for  the  as-deposited  and annealed Pd/Ru Schottky contacts is shown in Fig. 2. Capacitance-voltage    924  N. NANDA KUMAR REDDY, V. RAJAGOPAL REDDY   (C-V)  measurements  are  performed  at  a  frequency  of  1 MHz.  The  C-V relationship for Schottky diode is given by [4],   2212() biSdkTVVq C qN A  (5)  where  s is the permittivity of the semiconductor ( s   9.5  0), V is the applied voltage and A is the area of the diode. The x-intercept of the plot of (1/C2) versus V gives V0 and it is related to the built-in potential Vbi by the equation Vbi   V0   kT/q,  where  T  is  the  absolute  temperature.  The  barrier  height  is given  by  the  equation  ΦCV   V0 + Vn + kT/q,  here  Vn   (kT/q) ln(Nc/Nd).  The density of states in the conduction band edge is given by Nc   2(2 m kT/h2) 3/2, where  m*   0.22 m0  and  its  value  is  2.6   1018 cm  3  for  n-GaN  at  room temperature [5]. The carrier concentration (Nd) can be extracted from the slope  of  1/C2  versus  V plot  and  the  corresponding  values  are  3.84   1017 cm 3 for the as-deposited, 3.44   1017 cm 3 for 200  C, 2.59   1017 cm  3 for 300  C,  2.94   1017 cm  3  for  400  C,  2.77   1017 cm  3  for  500  C  and 3.92   1017 cm  3  for  600  C  annealed  samples.  The  measured  Schottky barrier height of the as-deposited Pd/Ru Schottky  contact is found to  be 0.79 eV.  For  the  samples  annealed  at  200,  300,  400,  500  and  600  C  the Schottky  barrier  heights  are  0.80 eV,  0.96 eV,  0.85 eV,  0.84 eV  and 0.72 eV, respectively.   Fig. 3 shows a plot of barrier heights and ideality factors of Ru/Pd/n-GaN Schottky diode as a function of annealing temperature. From Fig. 3, it is  also  evident  that  the  barrier  heights  ( I-V)  determined  from  I-V measurements  are  significantly  lower  than  those  obtained  from  C-V measurements  ( C-V).  The  difference  in  barrier  heights  obtained  from  I-V and C-V measurements may arise from the presence of a native oxide layer [6] or spatial inhomogeneity at the Ru/Pd/n-GaN interface [7].    Fig. 3 – Variation  of  the  barrier  heights  and  ideality  factors  with  annealing temperature for the Pd/Ru Schottky contacts on n-type GaN        THERMAL ANNEALING BEHAVIOUR ON ELECTRICAL…  925 4.  CONCLUSIONS  Rapid thermal annealing effects on the electrical properties of Ru/Pd/n-GaN Schottky  contacts  have  been  investigated  by  current-voltage  (I - V)  and capacitance-voltage  (C-V)  techniques.  As-deposited  Ru/Pd/n-GaN  Schottky diode  exhibits  barrier  height  of  0.67 eV  (I-V)  and  0.79 eV  (C-V), respectively. Experimental results indicated that the Schottky barrier height increased from 0.68 eV (I-V) and 0.80 eV (C-V) to 0.80 eV (I-V) and 0.96 eV (C-V)  as  the  annealing  temperature  is  varied  from  200  C  to  300  C  and decreased to 0.59 eV (I-V) and 0.72 eV (I-V) after annealing at 600  C. The variation of Schottky barrier heights with different annealing temperatures may be attributed to the interfacial reactions of Pd/Ru with GaN.  REFERENCES 1.  S.N. Mohammad, A. Salvador, H. Morkoc, Proc. IEEE 83, 1306 (1995). 2.  E.H. Rhoderick,  R.H. Williams,  Metal-Semiconductor  Contacts  (Oxford, Clarendon Press: 1988). 3.  M. Drechsler, D.M. Hofman, B.K. Meyer, T. Detchprohm, H. Amano, I. Akasaki, Jpn. J. Appl. Phys. 34, L1178 (1995). 4.  S.M. Sze, Physics of semiconductor devices (NewYork, Wiley: 1981). 5.  X.J. Wang, L. He, J. Electron. Mater. 27, 1272 (1998).  6.  P. Hacke, T. Detchprohm, K. Hiramatsu, N. Sawaki, Appl. Phys. Lett. 63, 2676 (1993). 7.  J.H. Werner, H.H. Guttler, J. Appl. Phys. 69, 1522 (1991). 

Metal-Semiconductor Contacts

Physics of semiconductor devices

Thermal annealing behaviour on electrical properties of Pd/Ru Schottky contacts on n-type GaN

SocioEconomic Challenges Journal (Sumy State University)

 
 
 
J. Nano- Electron. Phys. 
3 (2011) No1, P. 921-925 
 2011 SumDU 
(Sumy State University) 
 
 
921 
PACS numbers: 85.30.Hi, 85.30.Kk 
 
THERMAL ANNEALING BEHAVIOUR ON ELECTRICAL PROPERTIES 
OF Pd/Ru SCHOTTKY CONTACTS ON n-TYPE GaN 
 
N. Nanda Kumar Reddy, V. Rajagopal Reddy  
 
 Sri Venkateswara University,  
 Department of Physics, Semiconductor Devices Lab, Tirupati-517502, India 
 E-mail: nandasvu@gmail.com 
 
We have investigated the electrical properties of Pd/Ru Schottky contacts on n-GaN 
as a function of annealing temperature by current-voltage (I-V) and capacitance-
voltage (C-V) measurements. The Schottky barrier height of the as-deposited Pd/Ru 
contact is found to be 0.67 eV (I-V) and 0.79 eV (C-V), respectively. Measurements 
showed that the Schottky barrier height increased from 0.68 eV (I-V) and 0.80 eV (C-V) 
to 0.80 eV (I-V) and 0.96 eV (C-V) as the annealing temperature is varied from 200 C 
to 300 C. Upon annealing at 400 C and 500 C, the Schottky barrier height 
decreased to 0.73 eV (I-V) and 0.85 eV (C-V) and 0.72 eV (I-V) and 0.84 eV (C-V), 
respectively. It is noted that the barrier height further decreased to 0.59 eV (I-V) and 
0.72 eV (C-V) when the contact is annealed at 600 C. The change of Schottky barrier 
heights and ideality factors with annealing temperature may be due to the formation 
of interfacial compounds at the Ru/Pd/n-GaN interface. 
 
Keywords: SCHOTTKY BARRIER DIODE, THERMAL ANNEALING TEMPERATURE, 
I-V AND C-V CHARACTERISTICS, LEAKAGE CURRENT, BARRIER HEIGHT. 
 
(Received 04 February 2011, in final form 30 November 2011) 
 
1. INTRODUCTION 
 
Because of its wide band gap, high electron saturation velocity, large 
breakdown field and thermal stability, GaN is an attractive candidate for high 
temperature/high-power electronics [1]. Devices such as metal-semiconductor 
field effect transistors (MESFETs), high-electron-mobility transistors (HEMTs) 
and hetrojunction bipolar transistors (HBTs) have been fabricated on GaN [1]. 
In order to enhance the efficiency of these devices, thermally stable ohmic and 
Schottky contacts are required. A number of metals have been used as 
Schottky contacts to GaN by several research groups eg Pt, Au, Ni, Re and Ir. 
In the present work, we have fabricated and investigated thermal annealing 
temperature effects on the electrical characteristics of Pd/Ru Schottky 
contacts to n-type GaN. 
 
2. EXPERIMENTAL DETAILS 
 
In the present work, 2 m thick Si doped GaN films are used, which are 
grown by metalorganic chemical vapour deposition (MOCVD) on c-plane Al2O3 
substrates. The electron concentration obtained by means of Hall measure-
ments is  5.07  10 17 cm 3. The n-GaN layer is first ultrasonically degreased 
with warm trichloroethylene followed by acetone and methanol for 5 min 
each. This degreased layer is then dipped into boiling aqua regia 
 
 
 
922 N. NANDA KUMAR REDDY, V. RAJAGOPAL REDDY  
[HNO3 : HCL  1 : 3] for 10 min to remove the surface oxides and rinsed in 
deionized (DI) water. Ohmic contact is formed by depositing Ti (20 nm)/ 
Al (100 nm) on a portion of the sample by electron beam evaporation system. 
Rapid thermal annealing is conducted at 650 C for 3 min in nitrogen 
ambient. Subsequently, a Schottky metallization scheme is achieved by 
depositing Pd (20 nm)/Ru (30 nm) bilayer with a diameter of 0.7 mm 
through a stainless steel mask in an electron beam evaporation system at a 
vacuum pressure of 7  10 6 mbar. In order to study the thermal annealing 
effects on Pd/Ru Schottky contacts to n-GaN, samples were annealed at 
200 C, 300 C, 400 C, 500 C and 600 C for 1 min in N2 atmosphere in a 
rapid thermal annealing (RTA) system. Current-voltage (I-V) and 
capacitance (C-V) characteristics of the Schottky diode were measured at 
room temperature using Keithley source measuring unit (model No. 2400) 
and automated DLS - 83D spectrometer (SEMILAB, Hungary), respectively.  
 
3. RESULTS AND DISCUSSION 
 
We analyze the experimental I-V characteristics by the forward bias 
thermionic emission (TE) theory given as follows [2], 
 
 0 exp( )[1 exp( )]
qV qV
I I
nkT kT
 (1) 
 
Where 
 bo
q
I AA T
kT
* 2
0 exp( )  (2) 
 
here I0 is the saturation current, q is the electronic charge, A
* is the 
effective Richardson’s constant and equals to 26.4 A/cm2K2 for n-GaN [3], 
A is the effective diode area, k is the Boltzmann constant, bo is the 
Schottky barrier height and n is the ideality factor.  
 
 
 
Fig. 1 – I-V characteristics of Pd/Ru Schottky contact on n-GaN as a function of 
annealing temperatures 
 
The ideality factor is calculated from the slope of the linear region of the 
forward-bias lnI versus V plots and can be written from equation (1) as, 
 
  
 
 
 THERMAL ANNEALING BEHAVIOUR ON ELECTRICAL… 923 
 ( )
(ln )
q dV
n
kT d I
 (3) 
 
The Schottky barrier height ( bo) is determined from the extrapolated I0 and 
is given by, 
 
 
* 2
0
ln( )bo
kT AA T
q I
 (4) 
 
 Using linear curve fitting for the forward characteristics of lnI versus V 
data, the barrier height ( bo) and ideality factor (n) can be determined from 
the y-intercept and the slope of the fitted curves respectively. 
 From Fig. 1, the reverse leakage current value at – 2 V for the as-deposited 
contact is measured to be 3.79  10  6 A. For the samples annealed at 200 C, 
300 C, 400 C, 500 C and 600 C the reverse leakage current values are 
9.32  10  6, 3.76  10  9, 5.60  10  7, 9.29  10  7 and 4.02  10  4 A, respec-
tively. The smallest value of leakage current 3.76  10  9 A is observed for 
the contact annealed at 300 C. Calculations yielded a Schottky barrier 
height of 0.67 eV for the as-deposited Pd/Ru contact. The extracted barrier 
heights of the annealed Pd/Ru Schottky contacts are 0.68 eV for 200 C, 
0.80 eV for 300 C, 0.73 eV for 400 C, 0.72 eV for 500 C and 0.59 eV for 
600 C. The ideality factor of the as-deposited Ru/Pd/n-GaN Schottky 
contact is found to be 1.62. However, the ideality factor increased to 2.04 
upon annealing at 600 C. Ideality factors above unity has been attributed to 
interface states due to thin oxide layer between the metal and the 
semiconductor, including other contaminants, tunneling currents in highly 
doped semiconductors, image-force lowering of the Schottky barrier in 
electric field at the interface and generation-recombination currents with in 
the depletion region [2].  
 
 
 
Fig. 2 – Plot of 1/C2 versus V for the Pd/Ru Schottky contacts annealed at different 
temperatures 
 
A plot of 1/C2 as a function of bias voltage for the as-deposited and 
annealed Pd/Ru Schottky contacts is shown in Fig. 2. Capacitance-voltage 
 
 
 
924 N. NANDA KUMAR REDDY, V. RAJAGOPAL REDDY  
(C-V) measurements are performed at a frequency of 1 MHz. The C-V 
relationship for Schottky diode is given by [4], 
 
 
2 2
1 2
( )bi
S d
kT
V V
qC qN A
 (5) 
 
where s is the permittivity of the semiconductor ( s  9.5 0), V is the applied 
voltage and A is the area of the diode. The x-intercept of the plot of (1/C2) 
versus V gives V0 and it is related to the built-in potential Vbi by the equation 
Vbi  V0  kT/q, where T is the absolute temperature. The barrier height is 
given by the equation ΦCV  V0 + Vn + kT/q, here Vn  (kT/q) ln(Nc/Nd). The 
density of states in the conduction band edge is given by Nc  2(2 m kT/h
2) 3/2, 
where m*  0.22 m0 and its value is 2.6  10
18 cm  3 for n-GaN at room 
temperature [5]. The carrier concentration (Nd) can be extracted from the 
slope of 1/C2 versus V plot and the corresponding values are 3.84  1017 
cm 3 for the as-deposited, 3.44  1017 cm 3 for 200 C, 2.59  1017 cm  3 for 
300 C, 2.94  1017 cm  3 for 400 C, 2.77  1017 cm  3 for 500 C and 
3.92  1017 cm  3 for 600 C annealed samples. The measured Schottky 
barrier height of the as-deposited Pd/Ru Schottky contact is found to be 
0.79 eV. For the samples annealed at 200, 300, 400, 500 and 600 C the 
Schottky barrier heights are 0.80 eV, 0.96 eV, 0.85 eV, 0.84 eV and 
0.72 eV, respectively. 
 Fig. 3 shows a plot of barrier heights and ideality factors of Ru/Pd/n-
GaN Schottky diode as a function of annealing temperature. From Fig. 3, it 
is also evident that the barrier heights ( I-V) determined from I-V 
measurements are significantly lower than those obtained from C-V 
measurements ( C-V). The difference in barrier heights obtained from I-V 
and C-V measurements may arise from the presence of a native oxide layer 
[6] or spatial inhomogeneity at the Ru/Pd/n-GaN interface [7]. 
 
 
 
Fig. 3 – Variation of the barrier heights and ideality factors with annealing 
temperature for the Pd/Ru Schottky contacts on n-type GaN 
 
  
 
 
 THERMAL ANNEALING BEHAVIOUR ON ELECTRICAL… 925 
4. CONCLUSIONS 
 
Rapid thermal annealing effects on the electrical properties of Ru/Pd/n-GaN 
Schottky contacts have been investigated by current-voltage (I - V) and 
capacitance-voltage (C-V) techniques. As-deposited Ru/Pd/n-GaN Schottky 
diode exhibits barrier height of 0.67 eV (I-V) and 0.79 eV (C-V), 
respectively. Experimental results indicated that the Schottky barrier height 
increased from 0.68 eV (I-V) and 0.80 eV (C-V) to 0.80 eV (I-V) and 0.96 eV 
(C-V) as the annealing temperature is varied from 200 C to 300 C and 
decreased to 0.59 eV (I-V) and 0.72 eV (I-V) after annealing at 600 C. The 
variation of Schottky barrier heights with different annealing temperatures 
may be attributed to the interfacial reactions of Pd/Ru with GaN. 
 
REFERENCES 
1. S.N. Mohammad, A. Salvador, H. Morkoc, Proc. IEEE 83, 1306 (1995). 
2. E.H. Rhoderick, R.H. Williams, Metal-Semiconductor Contacts (Oxford, 
Clarendon Press: 1988). 
3. M. Drechsler, D.M. Hofman, B.K. Meyer, T. Detchprohm, H. Amano, I. Akasaki, 
Jpn. J. Appl. Phys. 34, L1178 (1995). 
4. S.M. Sze, Physics of semiconductor devices (NewYork, Wiley: 1981). 
5. X.J. Wang, L. He, J. Electron. Mater. 27, 1272 (1998).  
6. P. Hacke, T. Detchprohm, K. Hiramatsu, N. Sawaki, Appl. Phys. Lett. 63, 2676 (1993). 
7. J.H. Werner, H.H. Guttler, J. Appl. Phys. 69, 1522 (1991). 


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Thermal annealing behaviour on electrical properties of Pd/Ru Schottky contacts on n-type GaN

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