Nonstoichiometric deep levels in Mg-doped GaP epitaxial layers

We have investigated the relation between deep levels in Mg-doped p-type GaP liquid phase epitaxy (LPE) layers and stoichiometry of the surface of the substrates by PHCAP measurement. Concentration of a deep donor level at E_C−1.9–2.1 eV is higher in an n-type undoped GaP substrate annealed with applying phosphorus vapor pressure of 20 kPa than in sample annealed beneath a carbon cover. Next, Mg-doped LPE layers are grown on substrates that have been pre-annealed under phosphorus vapor pressure just before the growth. The densities of deep levels at E_V+0.85 and E_V+1.5 eV in long-time (2 h) pre-annealing sample are greatly decreased, but a deep level at E_C−1.9–2.1 eV shows opposite tendency. The latter is thought to be identical to a deep level detected in the substrate, probably phosphorus interstitial atoms.     

Controlled vapor-pressure heat-treatment effect on deep levels in liquid-encapsulated czochralski-grown GaP crystals

Photocapacitance (PHCAP) measurements have been carried out on GaP crystals grown by the liquid-encapsulated Czochralski (LEC) method with heat treatment under various phosphorus-vapor pressures at different temperatures. Electron traps of E_C−1.1 eV, E_C−1.6 eV, E_C−1.9 eV, and a hole trap of E_V+2.26 eV are mainly detected. The phosphorus-vapor pressure dependence of the E_C−1.9 eV trap density and their diffusion behavior indicate that they are interstitial phosphorus atoms. The densities of both E_C−1.1 eV and E_C−1.6 eV traps are strongly dependent on the shallow impurity concentrations. Moreover, the density of E_C−1.1 eV traps increases with increasing phosphorus-vapor pressure. From these results, we suggest that E_C−1.1 eV traps are the complexes of shallow donors and antisite phosphorus atoms. Deep-level densities in GaP crystals after annealing at 860°C or 960°C for 60 min are decreased almost one order of magnitude lower than those in untreated substrate crystals, which should have occurred via out-diffusion of interstitial phosphorus atoms. However, such an effect is not prominent for 800°C treatment for 60 min.     

Characterization of closed space vapor transport GaP epitaxial layers

The growth of homoepitaxial GaP layers using Te-doped GaP as source material has been obtained by the so-called closed space vapor transport technique. The photoluminescence study shows that these layers, when grown under optimized thermodynamical conditions, have both a large luminescence efficiency and the same optical quality as the ones obtained by liquid phase epitaxy. The variation of the luminescence properties with the conditions of growth has been investigated. Both electron paramagnetic resonance and deep level transient spectroscopy detect the presence of deep levels that are not observed in liquid phase epitaxy materials.     

Quantitative esr analysis of deep defects in LEC-grown GaP

Defects in LEC GaP have been analyzed quantitatively using an ESR method. Transition-metal impurities and native defects were studied in as-grown crystals. The antisite defect, P_Ga, and the isolated gallium vacancy, V_Ga, were emphasized since no other method has given definite, quantitative information on these defects. P_Ga antisites are present in some Cr-doped and some Zn-dopeii samples in concentrations high enough to affect their electrical properties. V_Ga was not detected in as-grown crystals indicating that V_Ga concentrations are smaller than the antisite concentrations at least in p-type material. Typical ESR detection., limits for transition-metal impurities are in the middle 10¹⁴ cm⁻³ range. The method is especially sensitive to Fe traces which were observed in all the samples studied. Work performed while the author was a visiting scientist at TAF Freiburg,     

Defect Engineering in Semiconductors: Manipulating Nonstoichiometric Defects and Understanding Their Impact in Oxynitrides for Solar Energy Conversion

Nonstoichiometric defects, as manifested by slight deviation of elemental compositions from chemical formulas, are common yet highly important in solid materials. Oxynitrides with a relatively large O/N ratio variation are theoretically predicted to change their electronic structures and charge transport behaviors with these nonstoichiometric defects. However, little experimental effort is devoted to understanding the impact of such nonstoichiometric defects regarding varied O/N ratios in oxynitrides for solar water splitting. The main reason is the lack of suitable oxynitride research models for aforementioned nonstoichiometric defect study without interference from other factors. Using TaON as a prototypical material, finely tuned O/N ratios can dramatically influence its photoresponse. In‐depth analysis further reveals a significant impact of nonstoichiometric defects (O/N ratios) on TaON's charge carrier densities, charge separation, and transport. Finally, manipulating nonstoichiometric defects of O/N ratios demonstrates its ability to control the space charge layer width and film conductivity of TaON photoanodes for high efficiency water splitting. Therefore, a fine understanding and control of nonstoichiometric defects would be highly important for future development of high efficiency oxynitrides for water splitting.     

Stoichiometry-dependent deep levels in undoped p-type Al0.38Ga0.62As grown by liquid phase epitaxy

Photocapacitance (PHCAP) and photoluminescence (PL) measurements were applied to unintentionally doped p-type Al_0.38Ga_0.62As grown by liquid phase epitaxy using the temperature difference method under controlled vapor pressure. PHCAP spectra revealed three dominant deep levels at E_v+0.9, E_v + 1.45, and E_v+1.96 eV, and a deep level at E_v+0.9−1.5 eV which was not neutralized by forward bias injection. These level densities increase with increasing arsenic vapor pressure and net shallow acceptor density. Furthermore, PL spectra reveal a deep level at 1.6–1.7 eV. The PL intensity of this deep level increases with increasing arsenic vapor pressure. These deep levels are thought to be associated with excess As.     

Real-time optical monitoring of epitaxial growth: Pulsed chemical beam epitaxy of GaP and InP homoepitaxy and heteroepitaxy on Si

We present a study of the real-time monitoring of the homoepitaxial growth of GaP, InP, and the growth of InP/GaP and GaP/Si(001) heterostructures, combining single wavelength p-polarized reflectance (PRS), reflectance-difference spectroscopy (RDS), and laser light scattering (LLS) during pulsed chemical beam epitaxy with tertiarybutylphosphine, triethylgallium, and trimethylindium sources. The growth rate and the bulk optical properties are revealed by PRS with submonolayer resolution over 1000A of film growth. The surface topography is monitored by LLS providing additional information on the evolution of the surface roughness as well as the nucleation/growth mechanism. The optical surface anisotropy, which is related to surface reconstruction and/or surface morphology, is monitored by RDS and compared with the results of PRS and LLS. The results are discussed with respect to the deposition kinetics, in particular as a function of the V:III flux ratio. The pulsed supply of chemical precursors causes a periodic alteration of the surface composition, which is observed as correlated periodic changes in the RD and PR signals, confirming the high sensitivity of both methods to surface chemistry.     

GaP:N绿色LEDn_2层载流子浓度的优化

在已报道的ｐ－ｎ２－ｎ１结的势垒分布计算的基础上，对该结构的浓度分布进行了计算。对于正偏情形，计入了ｎ２区产生的压降。考虑到ＧａＰ：ＮＬＥＤ发光区主要在ｐ区，注入效率γ＝ｊｎ/（ｊｎ＋ｊｐ），ｊｎ和ｊｐ分别为电子电流和空穴电流。ｐ区内的少子扩散可视为向无限远处的一维扩散；ｎ２区内外加正向偏压时电场不能忽略，空穴又被ｎ２－ｎ１结势垒阻挡（设被完全阻挡），则问题归结为求解有限厚度层中空穴的扩散和复合方程，由边界条件求出空穴扩散电流。将求出的电子扩散电流和空穴扩散电流代入注入效率的表达式即可求得γ。对在合理的参数值范围内的计算结果进行了讨论。分析表明：当ｎ２值在１０１５～１０１６ｃｍ－３范围内时，注入效率较高，与实验结果基本相符。     

Mobility vs thickness in n+-ZnO films: Effects of substrates and buffer layers

We review recent work on thickness effects in thin films of Ga- and Al-doped ZnO (GZO and AZO, respectively) grown by pulsed laser deposition (PLD), RF sputtering (RFS), or molecular beam epitaxy (MBE), and comprising four, distinct types of structures: (1) films grown directly on lattice-mismatched substrates; (2) films grown on buffer layers on lattice-mismatched substrates; (3) films grown on lattice-matched bulk substrates; and (4) homoepitaxial films with nearly perfect interfaces. Representative examples of each type include: 1a) PLD-GZO/Si; 1b) RFS-AZO/quartz; 2) RFS-AZO/ZnON/quartz; 3) PLD-GZO/bulk-ZnO; and 4) MBE-GZO/MBE-ZnO/GaN. Samples 1a, 1b, 2, and 3 can all be well described by a simple, phenomenological model for the thickness (d) dependence of sheet concentration n_s and mobility μ: n_s(d)=n(∞)(d - δd), and μ(d)=μ(∞)/[1+d*/(d-δd)], where n(∞) is the predicted volume carrier concentration at d=∞ (i.e., the bulk value), δd is the thickness of the dead layer (if any) between film and substrate, μ(∞) is the predicted mobility at d=∞, and d* is a figure of merit for the electrical properties of the interface. Samples 1a, 1b, 2, and 3, are well explained by the model with d*=23, 22, 7, and 3 nm, respectively. However, sample 4 does not obey the d* model at all and must be explained by entirely different physics. The films are characterized by Hall-effect, X-ray reflectance, X-ray photoelectron spectroscopy, reflectance, and transmittance measurements.     

Properties of gaas:si epitaxial layers grown in a multiwafer MOCVD reactor

We have studied the chemical and electrical properties of Si doped GaAs layers grown from SiH₄. - TMG - ASH₃ - H₂ at atmospheric pressure in a large scale metal-organic chemical vapor deposition (MOCVD) reactor. Excellent mobilities at the high doping levels imply doping efficiency in our reactor is higher than previously reported. We have measured extremely uniform doping and doping-thickness product of the Si doped epilayers. The importance of reactor configuration and the surface controlled growth reaction is illustrated by the relatively temperature independent dopant incorporation and the growth rate dependence of the GaAs on the bulk Fermi level.     

Effect of composition on deep levels in heteroepitaxial GexSi1−x layers and evidence for dominant intrinsic recombination-Generation in relaxed ge layers on Si

Electron traps, hole traps, and the dominant recombination-generation (R-G) centers have been investigated with deep level transient spectroscopy and current-voltage/temperature measurements in heteroepitaxial Ge_xSi_1-x alloys with x ranging from 0.15 to 1, grown on graded Ge_y.Si_1−y/Si substrates. For all samples with compositions x < 0.85, which retain the Si-like conduction band structure, we detect a dominant electron trap and R-G center whose activation energy is ΔE = 0.5 eV, independent of composition. This energy agrees with that of electron traps previously reported for plastically deformed (PD) Si, suggesting a connection to the Si-like band structure. This 0.5 eV level dominates the reverse leakage current over a wide range of growth and annealing conditions for the 30% Ge samples, indicating that the electronic state at ΔE = 0.5 eV is a very efficient R-G center, as would be expected from its midgap position. Alternatively, for strain relaxed, pure Ge (< 1), we detect electron traps at E_c − 0.42 eV and E_c − 0.28 eV, in agreement with the literature on PD Ge and Ge bicrystals. These energies are significantly different from those observed for x < 0.85, and we conclude that these changes in activation energy are due to changes in the conduction band structure for high Ge content. Moreover, in contrast with the Si-like samples (x < 0.85), the reverse leakage current in the relaxed Ge cap layer is not controlled by deep levels, but is rather dictated by intrinsic, band-to-band generation due to the reduced bandgap of Ge as compared to Si-like alloys. Only for reverse bias magnitudes which incorporate a significant portion of the graded buffer within the depletion region do R-G centers dominate the reverse leakage current. These results confirm the high quality of the strain-relaxed, pure Ge cap region which was grown on a Ge_ySi_1−y/Si step graded heterostructure (where y was increased from 0 to 1) by ultra high vacuum chemical vapor deposition. Finally, we report for the first time, what is apparently the dislocation kink site state at E_c − 0.37 eV, in a Ge_xSi_1−x alloy.     

An overview of early studies on persistent photoconductivity and other properties of deep levels in GaAsP: The effect of deep levels on light emitting devices

Laser studies carried out at the University of Illinois in 1966 showed that the shortest wavelength at which lasing could be achieved in GaAsP was dependent on then-type dopant. In particular sulfur doping was found to restrict lasing to longer wavelengths. It was suggested that this could be due to the effect of energy levels associated with the higher conduction band minima. This led in 1967 to an investigation of the effect of Te and S donor levels on the properties of GaAsP near the direct-indirect crossover. Samples throughout the composition range were studied using Hall effect measurements from 55 to 400° K and resistivity measurements under hydrostatic pressure between 0 and 7 kbar at 300, 195, and 77° K. The data on Te-doped samples fit the standard energy band models, but S doping was found to exhibit dramatic persistent photoconductivity and other compositional effects similar to the “DX-center” effects later observed in AlGaAs and other materials. This paper summarizes these results on GaAsP:S, and gives a brief overview of other early investigations in compound semiconductors where energy levels associated with higher lying minima were studied and where, in some cases, nonequilibrium effects were observed. A later study on GaAsP:S is also described which shows the effect of S-doping on LED performance. Finally, some of the implications that the existence of deep levels of this type have on light emitting device performance in other alloy systems is discussed.     

Effects of GaAs buffer layer and lattice-matching on deep levels in Zn(S)Se/GaAs heterostructures

The effects of GaAs buffer layer and lattice-matching on the nature of deep levels involved in Zn(S)Se/GaAs heterostructures are investigated by means of deeplevel transient spectroscopy (DLTS). The heterojunction diodes (HDs) where nZn(S)Se is grown on p⁺-GaAs by metalorganic vapor phase epitaxy are used as a test structure. The DLTS measurement reveals that when ZnSe is directly grown on a GaAs substrate, there exist five electron traps A-E at activation energies of 0.20, 0.23, 0.25, 0.37, and 0.53 eV, respectively. Either GaAs buffer layer and lattice-matching may reduce the incorporation of traps C, D, and E, implying that these traps are ascribed to surface treatment of GaAs substrate and to lattice relaxation. Concentration of trap B, which is the most dominant level, is proportional to the donor concentration. However, in the ZnSSe/GaAs sub. HD, another trap level, instead of trap B, locates at the almost same position as that of trap B, and it shows anomalous behavior that the DLTS peak amplitude changes drastically as changing the rate windows. This is explained by the defect generation through the interaction between sulfide and a GaAs substrate surface. For the trap A, the concentration is a function of donor concentration and lattice mismatch, and the origin is attributed to a complex of donor induced defects and dislocations.     

Characterization of deep centers in undoped semi-insulating GaAs substrates by normalized thermally stimulated current spectroscopy: Comparison of 100 and 150 mm wafers

A well-established characterization method for investigating deep traps in semi-insulating (SI) GaAs is thermally stimulated current (TSC) spectroscopy; however, TSC is not considered to be a quantitative technique because it involves carrier mobility, lifetime, and geometric factors, which are either unknown or poorly known. In this paper, we first show how to quantify a TSC spectrum, by normalizing with infrared (hv = 1.13 eV) photocurrent, and then apply this method (called NTSC) to study the lateral uniformity of the main deep centers across the diameters of undoped SI GaAs wafers. The wafers used in the study include both the standard 100 mm sizes and the new 150 mm variations, and are grown by both the low and high pressure liquid encapsulated Czochralski techniques. The results reveal that the 150 mm wafers have a worse NTSC uniformity for the main traps and a higher degree of compensation, as compared these parameters for the 100 mm wafers. In addition, nonuniformities related to the electric field effects on both the TSC spectrum and the low temperature photocurrent are found in the 150 mm wafer grown by the low pressure technique.