Residual Acceptors Research Articles

Influence of residual dopants to net doping concentration in N-type 4H-SiC films grown using high-speed wafer rotation vertical CVD method

In this study, the influence of residual dopants on the net doping concentration in n-type 4H-SiC epitaxial films grown at different N2 flow rates and C/Si ratios were investigated. By reducing the N2 flow rate, the influence of the residual donors on the net doping concentration was observed to become dominant for the films grown at low C/Si ratios and that of the residual acceptors on the net doping concentration becomes dominant for the films grown at high C/Si ratios. For the films grown at the middle C/Si ratio, an apparent proportional relation due to the compensation balance between the residual and intentional donors and the residual acceptors was observed in the N2 flow rate dependence of the net doping concentration. Furthermore, the decay curve of the net doping concentration observed in the C/Si ratio dependence is affected by the compensation balance between the intentional dopant concentration and the residual dopant concentration.

High breakdown voltage of AlGaAs/GaAs/AlGaAs diode achieved by balanced charges considering residual carbon impurity in hole and electron channels

High breakdown voltage (BV) AlGaAs/GaAs/AlGaAs diodes with a pair of hole and electron channels were studied taking account of the residual carbon impurity. The residual acceptor that would affect the charge balance was evaluated by separate growth. Utilizing the residual concentration, the acceptor concentrations (N A) were examined with a fixed donor concentration of 1.1 × 1012 cm−2. For N A of 0.9 × 1012 cm−2, the diode with 113 μm drift region length derived the highest BV of 1800 V. Temperature-dependent I–V measurements revealed the breakdown due to an avalanche multiplication. This implies a uniform electric field across the channels. Then, depletion of both channels was characterized by C–V measurements for obtaining capacitance shut-off voltages of the diodes. The lowest shut-off voltage was obtained for the 0.9 × 1012 cm−2 N A diode. This would be due to the simultaneous depletion of hole and electron channels, i.e. the identical net concentration for acceptor and donor.

Electrical and microscopic characterization of p+-type layers formed in HgCdTe by arsenic implantation

The Hall-effect (with mobility spectrum analysis) and capacitance measurements were combined with transmission electron microscopy for the study of molecular-beam epitaxy-grown n-type Hg1-xCdxTe (x = 0.22) films subjected to various operations used in the fabrication of p +-n photodiode structures. The operations included arsenic ion implantation, activation annealing and extra annealings aimed at investigation of the details of the p +-n junction formation. A detailed characterization of p+-type layers formed by the implantation was done. A high degree of arsenic activation by activation annealing was demonstrated, along with the annihilation of implantation-induced extended (dislocation loops) and quasi-point defects, and the disappearance of electrically active centres associated with them. The annealing was also shown to cause the activation of residual acceptors, changing the degree of electrical compensation in the n-‘base’ of some of the p +-n structures.

The effect of Te dopant on the optical and electrical properties of high-resistivity AlSb crystals

Aluminum antimonide (AlSb) has a great potential application in electronic devices used at high temperatures. However, the properties of available AlSb crystals were still far away from the theoretical ones due to the high density of variant defects, especially point defects. Here, an improvement in optical transmittance and resistivity of AlSb was achieved by doping the as-grown AlSb with appropriate content of Te, through which the absorption of a spin split-off-heavy-hole band transition near 1.65 μm disappeared and the resistivity was increased from 102 to 106 Ω cm. It was suggested that the residual acceptors with the binding energy EA of 37.4 meV were compensated by a Te donor with ED = 71 meV in the AlSb:Te crystal, according to the evaluation of the competing recombination (e, A0) and (Te0, A0). An energy band model was proposed to describe the recombination process and the underlying mechanism. The photo-response of the high resistance AlSb:Te crystal, illuminated by a pulsed LED light in the visible light range, revealed that the ratio of photocurrent to dark current was 128, indicating a great potential application for room-temperature photo-detection.

Luminescence spatial characteristics of ZnSe:Fe

Two types of thermal-diffusion-based technological routes have been realized to obtain ZnSe:Fe plates demanded for laser application in the mid-IR spectral range. For the technological routes used, the iron doping process is shown to lead to the formation of auxiliary luminescence bands including (1) luminescence of residual tetrahedral donors and acceptors; (2) unknown bound exciton line located at 459 nm; (3) broad bands with maxima at 490, 520, 670, and 820 nm; and (4) IR band at 960 nm previously assigned to centers related to transition metals. Using the measurements of photoluminescence under conditions of two-photon excitation and measurements of low-temperature microphotoluminescence, the intensity profiles of the bands have been shown to be uncorrelated with the concentration profile of the optically active (Fe2+) iron. This trend does not depend on the technological routes used. Thus, heavily Fe doped areas, as well as adjacent regions in ZnSe:Fe plates produced by thermal-diffusion-based techniques, contain auxiliary centers governing luminescence in visible and near IR spectral ranges. These centers are not directly related to the optically active (Fe2+) iron.

Probing of single acceptors in a wide ZnSe-based quantum well via optical spectroscopy

Using low-temperature (5 K) micro-photoluminescence measurements of a ZnSe/ZnMgSSe quantum well, isolated (quantum) emitters produced by interacting acceptor and hydrogen-like donor have been found and studied. These emitters enable optical probing of a single acceptor (donor) center if the nature of the donor (acceptor) constituting the distant donor-acceptor pair is known. Here we demonstrate the simplest realization of proposed optical probing: frequencies of optically active phonons are successfully determined for different types of single residual acceptors in a ZnSe-based quantum well.

Development of n-type Te-doped GaSb substrates with low carrier concentration for FPA applications

Undoped GaSb is p-type with the residual acceptor concentration of about 1e17cm−3 due to the gallium vacancies and gallium in antimony site. Counter-doping of GaSb with low level of Te can reduce the net carrier concentration resulting in higher optical transparency in a broad IR spectral range. In this work, the carrier concentration, mobility and sheet resistance of n-type and p-type Te-doped GaSb substrates were measured using Hall method at 300K and 77K. The Hall carrier concentration data at 300K were correlated with the absorption coefficients of GaSb in the IR spectral range. An empirical relationship between these values was established. Based on this correlation, we discuss application of FTIR spectroscopy for non-destructive optical screening of the substrates that allows construction of the carrier concentration distribution map across GaSb wafers. Investigations of the electronic properties of the low-doped p-type and n-type GaSb substrates upon cooling down to 77K indicate the reduction of the hole carrier concentration background for both GaSb types. This is evident from the decrease in the Hall-measured carrier concentration for p-type GaSb. For n-type GaSb, an increase in the carrier concentration is observed due to the reduction of the hole carrier concentration background.

Electrical Properties of Diamond Platinum Vertical Schottky Barrier Diodes

Electrical and photoelectrical properties of Pt vertical Schottky barrier diodes prepared on thick lightly-doped CVD diamond grown on heavily-doped IIb HPHT substrates were studied. The films were p-type, with the concentration of residual boron acceptors in the mid-1014 cm-3. The Schottky barrier heights determination from photocurrent spectra measurements revealed the presence of two barrier values of 1.45 eV and 2 eV. The former dominated the temperature dependence of the forward current, the latter determined the voltage cut-off in C-V measurements. The results are explained by difference in oxygen termination conditions. The higher barrier is attributed to oxygen-related states at the terminated surface, the lower barrier is associated with native defects.

Device design of GaSb/CdS thin film thermal photovoltaic solar cells

Enthusiasm in the reflearch of thermo-photovoltaic (TPV) cells has been aroused because the low bandwidth semiconductors of III-V family are coming into use. GaSb, as a member of III-V family, has many merits such as high absorption coefficient, and low band gap of 0.725 eV at 300 K etc.. At preflent thermo-photovoltaic cells are usually based on GaSb wafer, and it can be manufactured by the vertical Bridgeman method. Thermo-photovoltaic cell based on GaSb films is one of the effective ways to reduce the cost of the thermo-photovoltaic system. GaSb polycrystalline films can be grown by physical vapor deposition (PVD) which has advantages in using fewer materials and energy, and also in doing little harm to the environment. Because of residual acceptor defects VGaGaSb, GaSb thin film is usually of p-type semiconductor. So we should find n-type semiconductor material to form pn junction. We choose CdS as the emission layer of a cell structure. CdS belongs to n-type semiconductor with a narrow band gap of 2.4 eV and high light transmissivity. CdS thin film grown by chemical bath deposition (CBD) has passivation properties for GaSb. CdS layers can remove native oxides from GaSb surface and reduce the surface recombination velocity of GaSb. This paper focuses on theoretical analysis of GaSb/CdS thin film photovoltaic structure. By way of AFORS-HET simulation, we analyze the defect state density and interface density in GaSb and CdS, and their effects on cell performance. According to the simulation, the defect density in GaSb absorption layer is the very important factors that affect cell performance. When GaSb defect increases, the major factor to affect the cell is the fill factor that leads to low efficiency. On condition that thereflexists high GaSb defect density, the thickness of GaSb should be kept at 1000 nm. GaSb with a thickness above 1000 nm can bring about a high recombination rate, which reduces the efficiency of the cell. As an emission layer, the defect density in CdS should not affect the cell performance obviously. When the increase of CdS defect density is of four orders of magnitude, the cell efficiency is only decreased by 0.11%. In order to demonstrate the interface between GaSb and CdS, we use an inversion layer n-GaSb according to the passivation properties of CdS thin film grown on GaSb. When the defect density of inversion layer increases, the efficiency of the cell will decrease rapidly. And the GaSb/CdS structure will act as a resistance when the defect density in the inversion layer reaches 1020 cm-3. So the defect density in GaSb layer and the interface is the very factor to affect thermo-photovoltaic performance.

Optical and magnetic resonance studies of Be-doped GaN bulk crystals

Low temperature photoluminescence (PL) and optically-detected magnetic resonance (ODMR) at 24GHz have been performed on a set of Be-doped GaN bulk crystals grown by the high pressure nitrogen solution method. Most notably, these crystals exhibit an intense 2.25eV “yellow” PL band that is currently of high interest for use as a converter in white light-emitting-diode applications. In addition, the samples investigated in this work are found to emit a broad “violet” PL band of varying intensity with peak energy near 3.16eV that is shown to be associated with a high concentration of residual Mg acceptor impurities as supported by secondary ion mass spectroscopy measurements. ODMR angular rotation studies obtained on the “yellow” and “violet” emission bands revealed two strong signals in each case and helped to provide information on the nature of these recombination processes. One feature, common to the ODMR of each PL band, has Zeeman splitting g-values of ~1.95 and is ascribed to residual Si and O shallow donor impurities. The second ODMR signal observed on the 2.25eV “yellow” PL exhibits nearly isotropic g-values of ~2.0 characteristic of deep centers in GaN and is tentatively associated with Be-related deep acceptors. In contrast, the second ODMR feature found on the 3.16eV “violet” PL has a larger degree of anisotropy with g║~2.1 and g⊥~2.0 and are similar to the g-values assigned previously to Mg shallow acceptors from ODMR studies of highly Mg-doped GaN heteroepitaxial layers with large degree (≥10%) of compensation.

Unintentional high-density p-type modulation doping of a GaAs/AlAs core-multishell nanowire.

Achieving significant doping in GaAs/AlAs core/shell nanowires (NWs) is of considerable technological importance but remains a challenge due to the amphoteric behavior of the dopant atoms. Here we show that placing a narrow GaAs quantum well in the AlAs shell effectively getters residual carbon acceptors leading to an unintentional p-type doping. Magneto-optical studies of such a GaAs/AlAs core-multishell NW reveal quantum confined emission. Theoretical calculations of NW electronic structure confirm quantum confinement of carriers at the core/shell interface due to the presence of ionized carbon acceptors in the 1 nm GaAs layer in the shell. Microphotoluminescence in high magnetic field shows a clear signature of avoided crossings of the n = 0 Landau level emission line with the n = 2 Landau level TO phonon replica. The coupling is caused by the resonant hole-phonon interaction, which points to a large two-dimensional hole density in the structure.

Photoluminescence of inversion electrons with carbon acceptors in a single modulation-doped AlxGa1−xAs/GaAs heterostructure

The photoluminescence of inversion electrons with carbon acceptors in a modulation-doped Al${}_{x}$Ga${}_{1\ensuremath{-}x}$As-GaAs single heterostructure was studied for different temperatures and excitation intensities. Despite the essentially homogeneous distribution of residual acceptors in the GaAs buffer, well-resolved luminescence profiles are observed for the ground and first excited two-dimensional electron subbands. This is due to the fact that the inversion layer interface potential restricts the overlap of the electron and acceptor wave functions to limited spatial regions close to the Al${}_{x}$Ga${}_{1\ensuremath{-}x}$As-GaAs interface, where the transition energies are weakly dispersive. A simple model will be presented for the calculation of the luminescence line shape that allows a quantitative analysis of the experiment.

Contact vs bulk effects in N-semi-insulating-N and P-semi-insulating-P diodes

We analyse the charge transport mechanism in semi-insulating (SI) materials for N-SI-N and P-SI-P structures. The SI layers are large band gap semiconductors obtained by deep levels compensation of residual shallow donors or acceptors. A preceding theory is extended to the case of two, donor or acceptor, deep compensating levels [1]. The conduction mechanism is complex: ambipolar transport, heavy recombination, space charge effect and we show that the current is controlled by both bulk and interface effects at the reverse biased N-SI or P-SI junction. We develop a simple model, without the usual assumption of space charge neutrality, valid up to the beginning of one carrier space charge current. We show that a linear relation exists between the bulk excess free carrier densities and that depending on the value of a quantity M, given as a function of the deep levels electrical parameters, energy position, concentration and capture cross sections, as well as the dopants concentrations, the conduction mechanism is either contact controlled showing a pronounced current saturation effect or bulk controlled with one and for long sample, two, quasi linear J–Va relationship. Numerical modelisations of the drift–diffusion transport model confirm these analytical results for the case of one or two compensating deep levels. GaAs (SI) or InP (SI) layers are used for their high resistivity and insulating properties in FET technology, in buried heterostructures, diode laser and in radiation detectors technologies. These results are of importance for the interpretation of conductivity and Hall Effect measurements and explain the parasitic side-gating effect in GaAs or InP MESFET’s.

Influence of Growth Conditions and Doping on Physical Properties of Gallium Antimonide Single Crystals

Gallium antimonide (GaSb) single crystals were grown by modi ed Czochralski method integrated with in situ synthesis in a owing atmosphere of pure hydrogen. The in uence of charge material purity as well as other technological parameters on GaSb crystals quality was investigated. High purity undoped GaSb single crystals were grown with residual acceptors concentration < 1.4 × 10 cm−3 and high mobility ≈ 690 cm/V s (at 300 K). P -type GaSb crystals were doped with silicon (carrier concentration up to 2 × 10 cm−3) and with zinc (up to 1× 10 cm−3). Tellurium doped n-type GaSb single crystals were obtained with concentration up to 2× 10 cm−3. Electrical parameters were investigated by the Hall measurements (300 K and 77 K). Temperature dependent Hall measurements (10 ÷ 300 K) were used to compare the quality of undoped GaSb (obtained from Sb of di erent purity). Dopant concentration was estimated by glow discharge mass spectroscopy analysis. Axial and radial distribution of carrier concentration were investigated especially for Te-doped crystals (low segregation coe cient of Te in GaSb). Great contribution of compensation and self-compensation mechanisms is shown especially for the beginning part of grown crystals and for low Te-doping level. Radial distribution of physical properties for crystals grown in ⟨100⟩ direction is not axisymmetrical especially for doped GaSb crystals.

Tailoring the photoluminescence characteristics of p-type GaSb: The role of surface chemical passivation

We report a systematic study on modulating optical properties in gallium antimonide (GaSb) with the sulfur surface passivation. Compared with the pristine sample, the intensity of the photoluminescence (PL) emission from the passivated GaSb was dramatically enhanced about 15 times at room temperature. The temperature-dependent surface states were further investigated via the low temperature PL spectroscopy. The bound-edge-related transition emission (BE4) at 795meV and the residual acceptor associated emission located at 777meV were identified clearly at 10K. The observed passivation effects on the photoluminescence characteristics evolution of GaSb are discussed in detail.

Effects of Growth Temperatures on Crystal Quality of GaN by Vapor Phase Epitaxy Using GaCl3 and NH3

Thick epitaxial growth of GaN by vapor phase epitaxy (VPE) is an indispensable technique to form GaN substrates which have been commonly used for blue-violet GaN-based lasers with sufficiently long lifetime. Although the growth has been established in view of the mass-production of the GaN substrate, the crystal quality has never been sufficiently correlated with the growth conditions. In this paper, the effects of the growth temperatures on the crystal quality of VPE-grown GaN films are studied in detail. It is noted that the GaN films are grown by a single-zone VPE using externally placed GaCl3 and NH3 as source precursors. The growth exhibits the growth rate of 14 µm/h at highest and far higher growth rate is possible by increasing the carrier flow rate and/or temperature of the GaCl3. The experimental results reveal that the surface morphology and optical properties are strongly dependent on the growth temperature. At around 975 °C with very narrow temperature window, very smooth surface together with a very sharp photoluminescence (PL) peak originating from bound excitons is observed. Growth at lower temperatures than the optimized window results in rough surface with many pits on it. Higher temperature results in many cracks and peeling-off on the surface with the sign of three-dimensional growth. Peaks originating from residual acceptors are dominant in the PL spectra of the films with rough surfaces grown at higher or lower temperatures from the optimized window. Flattening the surface of GaN at the optimized temperatures by the enhanced lateral growth is essential to grow thick GaN by the VPE with good crystalline quality free from the incorporation of the residual acceptors.

Correlation of residual impurity concentration and acceptor electron paramagnetic resonance linewidth in isotopically engineered Si

Electron paramagnetic resonance (EPR) experiments on boron acceptors in isotopically engineered 28Si samples with different degrees of chemical and isotopic purity are reported. The strong suppression of isotope-induced broadening effects in this material allows a direct observation of the linear correlation between the width of the inter-subband Δm = 1 EPR line and the concentrations of carbon, oxygen, and boron point defects down to a total concentration of ≈2 × 1015 cm−3. When the impurity level is decreased further, the linewidth does not fall below 2.3 ± 0.2 mT, for which we discuss possible origins.

Characterization of 6H-SiC Single Crystals Grown by PVT Method Using Different Source Materials and Open or Closed Seed Backside

n- and p-type 6H-SiC single crystals grown by PVT method using different charge materials – poly-SiC sinter or fresh SiC powder – have been studied. An open or closed seed backside during the growth processes have been applied. In the former, a distinct decrease backside etching of the seed was observed. Crystals have been extensively characterized with respect to their purity, quality and electrical properties using complex experimental methods. For the n-type boule an axially and radially homogeneous resistivity ~0.11 cm at 300 K was observed. Electrical properties of the p-type crystal, i.e., high room-temperature resistivity of 239 cm, were affected by compensation effects between residual donors (nitrogen and oxygen) and acceptors (mainly boron).

Theoretical and numerical investigations of carriers transport in N-semi-insulating-N and P-semi-insulating-P diodes – A new approach

A new and simple theoretical model is developed for ambipolar transport in compensated N-semi-insulating (SI)-N or P-SI-P diodes and is validated with exact J–Va characteristics obtained through numerical modelisation. The electrical parameters used correspond to SI GaAs layers, but these results are valid for other compounds such as SI InP and InGaP. A relation between the bulk non-equilibrium excess carrier concentrations, valid for low and intermediate applied voltage, is first established. For a deep donor (Nt) compensating a residual shallow acceptor (NA): (n-ne)≈τntτptNt-NANA(p-pe), where ne and pe are the thermal equilibrium free carrier densities in the SI layer, τnt, τpt and n1t, p1t are the familiar Shockley–Read–Hall (SRH) parameters of the deep trap. This relation represents an extension of the well known quasi space charge neutrality condition: (n−ne)≈(p−pe) valid for extrinsic semiconductors. We show then that a linear J–Va relationship is observed in N-SI-N diodes when Mt(=NA(Nt-NA)τntτptp1tn1t)<1 and in P-SI-P diodes when: Mt>1. The quasi totality of the applied voltage Va is lost across the SI layer and the electric field is constant (E≈Va/LSI). Mt<1 characterizes a SI(N−) layer where a strong hole depletion (p≈0) across the SI bulk is associated to an “ohmic” electron current where n is constant but such that n<ne. Mt>1 characterizes a SI(P−) layer where forn≈0, p<pe. On the other hand, the J–Va characteristics of N-SI(P−)-N diodes and P-SI(N−)-P diodes show a saturation effect. Most of the applied voltage is now lost across the reverse biased contact and the electric field is low across the SI layer. For Mt values close to 1, we switch from a linear to a saturation regime. Equivalent relations are given for a deep acceptor compensating a shallow donor. We present results for short SI layers having lengths LSI in the micrometer range and of the order or inferior to the ambipolar diffusion length LDa, such layer are used as insulating layers in buried heterostructures for diode laser technology, as well as for long SI layers, LSI≫LDa, as used in radiation detectors technology.

Doping, compensation, and photosensitivity of detector grade CdTe

We studied the resistivity, photosensitivity, photoluminescence, and surface photovoltage of CdTe crystals doped with Ge or Sn to extend our knowledge of the influence of the deep-donor level on compensation and afterglow effects. We demonstrated a strong correlation between photosensitivity caused by photoelectrons with Fermi-level variations near the GeCd0/2+ or SnCd0/2+ energy levels. Surface photovoltage measurements confirmed that when the concentration of residual acceptors varied along the direction of growth, then trapping conditions dramatically changed as a defect was converted from a neutral state to doubly charged positive one.