EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Benndorf, Gabriele

Gottschalch, Volker

Krahmer, Claudia

Leibiger, Gunnar

Riede, Volker

ESE - Salento University Publishing

Si− and Zn−doping of lattice matched BxInzGa1−x−zAs− and InxGa1−xNyAs1−y−layersGunnar Leibiger 1) ,  Claudia Krahmer 1) ,  Volker Gottschalch 1) ,  Gabriele Benndorf 2) ,  Volker Riede 2)1) Dept. of Inorganic Chemistry/Semiconductor Chemistry Group/University of Leipzig/Linnestr. 3, 04103 Leipzig, Germany ,  2) Dept. ofExperimental Physics II/University of Leipzig/Linnestr. 5, 04103 Leipzig, GermanyIntroductionInxGa1−xNyAs1−y−alloys have attained great attention in the past few years due to the possibility of lattice matchedor strained growth on GaAs substrates in combination with a large reduction of the band−gap energy withincreasing nitrogen incorporation [1]. Highly strained InxGa1−xNyAs1−y−layers with large In contents have beenused as active material in 1.3−1.5 µm laser diodes [2]. Lattice matched layers are of high interest forhigh−efficiency multi−junction solar cells [3]. The new BxInzGa1−zAs−material system, which is largely unknown,offers new possibilities in band−gap engineering and strain reduction [4,5].  The growth of lattice matched BxInzGa1−zAs−layers on GaAs has also been demonstrated introducing the material as another candidate forsolar−cell applications [4,5].  Systematic doping studies are prerequisite for application of both materials in solarcells or detector structures. However, systematic doping investigations of BxInzGa1−zAs and        InxGa1−xNyAs1−yusing metalorganic vapour−phase epitaxy (MOVPE) are either absent or very rare, respectively [6]. In this work, we investigate the Si− and Zn−doping of lattice matched BxInzGa1−zAs− and   InxGa1−xNyAs1−y−layers usingMOVPE and disilane and diethylzinc as doping precursors. All layers were characterized with high−resolutionx−ray diffraction, photoluminescence (PL), Hall−measurements and infrared spectroscopic ellipsometry (IR−SE).ExperimentalBxInzGa1−zAs− and  InxGa1−xNyAs1−y−layers have been grown lattice matched on (001)−GaAs substrates at 550°Cand 560°C, respectively using low−pressure (ptot = 50 mbar) MOVPE (AIX200). The total flow into the horizontalreactor amounted to 7ssl and the growth rate was ~ 800 nm/min. Triethyl boron (TEB), (1,1)−dimethyl hydrazine(DMHy), trimethyl gallium (TMGa), trimethyl indium (TMIn), diethyl zinc (DEZn) and disilane (500  ppm in H2)were used as B−, N−, Ga−, In−, Zn− and Si−precursors, respectively.  Tertiarybutyl arsine (TBAs) and arsine wereused as As−precursor for the growth of  InxGa1−xNyAs1−y and BxInzGa1−zAs, respectively.  The nitrogen− andindium compositions of the approximately 1 µm thick, lattice matched InxGa1−xNyAs1−y−layers can be estimated toy = 0.016 and x = 0.047 and the band−gap energy is ~1.08 eV. The compositions and the band−gap energy of thelattice matched  BxInzGa1−zAs−layers (d ~ 1 µm) amount to x @ 0.027, z @ 0.06 and Eg = 1.36 eV. The partialpressures of TMIn, TMGa, DMHy, and TBAs, used for the growth of InxGa1−xNyAs1−y, were    1.158*10−4,2.6980*10−3, 1.9*10−1, and 1.293*10−2  mbar, respectively. The partial pressures of TEB, TMIn, TMGa, andarsine, used for the growth of BxInzGa1−zAs,  amounted to 4.95*10−3 ,  1.365*10−4 ,  2.7146*10 −3 ,and  7.117*10−1  mbar, respectively.ResultsNominally undoped InxGa1−xNyAs1−y−layers were found to be p−type with free hole concentrations    ranging from~ 6×1016 cm−3 (V/III = 25) to ~ 2×1016 cm−3 for V/III−rations above 100. In comparison, the background−dopinglevel was drastically reduced for the   BxInzGa1−zAs−layers, for which free electron concentrations of 1010 cm−3(V/III = 41)  and 5×1013 cm−3 (V/III = 8) have been measured. These values are surprisingly low in view of therelatively large boron−carbon bondstrength.Fig. 1 (a) shows the free carrier concentrations of the Si−doped InxGa1−xNyAs1−y− and   BxInzGa1−x−zAs − layers resulting from the Hall−measurements as a function  of the   normalized  partial pressure of disilane in thegasphase.  With increasing Si−incorporation, the p−type  InxGa1−xNyAs1−y−layers become semiinsulating and for10th European Workshop on MOVPE, Lecce (Italy) 8−11 June 2003 PS.IV.08Gunnar Leibiger et al pdisilane/pgroup−III−values above 10−3, n−type conduction is obtained with a maximum electron concentration of~6×1018 cm−3 for pdisilane/pgroup−III = 0.1. For normalized disilane−partial pressures between 0.007 and 0.07, theincrease of the electron concentration is approximatively linear  and a Si−distribution coefficient kSi of ~0.004 canbe estimated. Please note that in thisapproximation autocompensation effects and the formation of Si−precipitates (both observed in Si−GaAs [7]) areneglected. For BxInzGa1−zAs, the Si−doping efficiency is clearly increased compared to InxGa1−xNyAs1−y (for2.2×10−5 < pdisilane/pgroup−III < 5.5×10−4), which can be understood by the smaller compensation ratio inBxInzGa1−zAs. The electron concentration increases linearly with increasing pdisilane/pgroup−III−ratio and aSi−distribution coefficient kSi of 0.075 can be estimated under the same assumptions as for InxGa1−xNyAs1−y. Thisvalue is significantly enhanced compared to InxGa1−xNyAs1−y (kSi ~ 0.004), which might be partly due to theincreased autocompensation ratio and/or tendency for formation of Si−precipitates for higher Si−concentrations[7]. Incorporation of Zn resulted in p−type conduction for both materials, InxGa1−xNyAs1−y and BxInzGa1−zAs (Fig. 1(b)). In Ref. 6, n−type conduction was obtained for Zn−doped          InxGa1−xNyAs1−y using MOVPE under similargrowth conditions.  The origin of this puzzling difference remains unsolved and needs further clarification. ForInxGa1−xNyAs1−y, we obtain linearly increasing hole concentrations with increasing pDEZn/pgroup−III−ratio up to thehighest obtained value of  p ~ 3×1018 cm−3 and a Zn−distribution coefficient kZn of ~ 0.012 can be estimated. Incomparison, the Zn−doping efficiency is drastically enhanced in BxInzGa1−zAs. Assuming that all Zn−atoms areincorporated on group−III lattice sites, a Zn−distribution coefficient kZn of ~ 0.4 can be estimated for pDEZn/pgroup−III−ratios between 0.001 and 0.03, where the incorporation behaviour is approximately linear.Different surface reconstructions and the higher As/group−III−ratio in  BxInzGa1−zAs leading to a higher number ofgroup−III vacancies might explain the observed differences between the two materials.Fig.  2 shows the Hall−mobil i t ies  of  the Si− (a)  and Zn−doped (b)  InxGa1 − xNyAs1 − y− (squares)and BxInzGa1−zAs−layers (crosses) as a function of the carrier concentration.  Generally, the mobilities of theBxInzGa1−zAs−layers are clearly enhanced (by a factor 2−5) compared to the corresponding values forInxGa1−xNyAs1−y, which can be explained by the lower compensation ratio in  BxInzGa1−zAs. For n−type layers,there is a trend of increasing mobilities with decreasing electron concentrations for high Si−compositions  (Fig. 2(a)). This can be understood by the decreasing number of ionized impurities with decreasing Si−concentration. Thesaturation or even decrease of the mobilities in the low−concentration region hints at a second process, possibly the10th European Workshop on MOVPE, Lecce (Italy) 8−11 June 2003 PS.IV.08Gunnar Leibiger et al interaction of the Si−dopants with other defects, which mayalso  be concluded from PL−experiments (see below).   The mobilities of both types of Zn−doped layers decreasewith increasing Zn−concentration due to enhanced ionized−impurity scattering and are generally lower than themobilities of the Si−doped samples, which is due to the higher valence−band effective masses (Fig. 2 (b)).For the Si−doped layers, infrared spectroscopic ellipsometry was used to derive mobilities and effective massesusing the Hall−concentrations as input parameter. Hall−mobilities and optically determined mobilities show ingeneral the same trends (Fig. 3 (a))  and the effective masses increase for both materials with increasing carrierconcentration (Fig. 3 (b)), which is an indicator of the nonparabolity of the conduction bands. The room temperature PL−spectra of both materials are largely influenced by Si−doping as shown in Fig. 4. For   InxGa1−xNyAs1−y  (a)  and BxInzGa1−zAs  (b), the maximum PL−intensities  increase with increasing doping levelup to ~ 1018 cm−3. This effect is possibly caused by the interaction of the Si−dopants with other defects. For higherdoping levels, the PL−intensities decrease in  InxGa1−xNyAs1−y, which can be explained by defect creation due to10th European Workshop on MOVPE, Lecce (Italy) 8−11 June 2003 PS.IV.08Gunnar Leibiger et al formation of Si−precipitates as observed in Si−GaAs [7].AcknowledgementThis work was supported by BMBF under contract 03WKI07.  We further acknowledge Mrs. U. Teschner and Dr.H. Herrenberger for considerable experimental support.References[1] M. Kondow et al., Jpn J. Appl. Phys. 35 (1996) 1273.  [2] K. Nakahara et al., IEEE Photonics Technol. Lett. 10 (1998) 487.[3] D. J. Friedman et al., J. Cryst. Growth 195 (1998) 409.[4] J. F. Geisz et al., Appl. Phys. Lett. 76 (2000) 1443.[5] V. Gottschalch et al., J. Cryst. Growth 248 (2002) 468.[6] K. Volz et al., J. Cryst. Growth 248 (2002) 451.[7] X. Tang et al., J. Cryst. Growth 98 (1989) 827.10th European Workshop on MOVPE, Lecce (Italy) 8−11 June 2003 PS.IV.08Gunnar Leibiger et al 

Si- and Zn-doping of lattice matched BxInzGa1-x-zAs- and InxGa1-xNyAs1-y-layers

Università del Salento: ESE - Salento University Publishing

English

Newbold, Lynne

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