Isotype Heterojunction Research Articles

Carrier injection in semiconductors with position-dependent band structure: Electron-beam-induced current at heterojunctions

We present simulations of external carrier injection in semiconductors with position-dependent band structure by numerical solution of the current transport equations across the device. As a model case, we examine electron-beam-induced current at heterojunctions. Our treatment is applicable to an arbitrary position-dependent band structure, and any form of external generation, and allows for an external bias applied to the device. The quality of ohmic contacts is taken account of by realistic boundary conditions. For excitation by an electron beam at a GaAs/AlGaAs n-N isotype heterojunction, we present band profiles, as well as linescans, of electron-beam-induced current and voltage. The influence of external generation rate, interface grading, and contact quality is investigated. The latter is shown to be important in epitaxial layers which are only a few minority-carrier diffusion lengths thick. The saturation behavior of the signal with increasing injection level is studied as a function of device parameters, and the applicability of such measurements to the determination of band offsets is critically discussed.

Measurement of the valence-band discontinuities for molecular organic semiconductor/inorganic semiconductor heterojunctions

Using the temperature dependence of the forward-biased current-voltage characteristics as well as internal photoemission, we directly measure the barrier potential and valence-band discontinuity energy (ΔEv ) of isotype heterojunctions formed between thin films of the crystalline organic semiconductor: 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and p-Si. We find ΔEv =(0.50±0.02) eV. This, to our knowledge, is the first report of a measurement of a band discontinuity energy between a crystalline organic semiconductor and an inorganic semiconductor. These results are consistent with predictions of a current model involving diffusion and drift in the organic-on-inorganic (OI) semiconductor device. This model is employed to calculate ΔEv using previously obtained barrier energies for several different PTCDA/inorganic semiconductor devices. In all cases, values of the barrier diffusion potential and ΔEv are considerably smaller than apparent barrier energies obtained previously using pure thermionic emission theory to explain transport of charge over the OI barrier.

Photocollection efficiency of GaAs/AlAs/GaAs p +-i-n and n +-i-p photodiodes

Measurements of the photocollection efficiency of GaAs/AlAs/GaAs diodes are reported. The quantum efficiency measurements demonstrate that the conduction band discontinuity between AlAs and GaAs is small — most of the band gap discontinuity is accommodated by the valence band. These results suggest that series resistance may be a problem in p-type heteroface cells (owing to the large majority carrier barrier); n-type heterofaces should display low series resistance due to the isotype heterojunction, even when the junction is not intentionally graded.

Electrically active defects in liquid phase epitaxial interfaces in the In 0.53Ga 0.47As/InP system

The electrical properties of liquid phase epitaxial interfaces in the lattice-matched In 0.53Ga 0.47As/InP system were studied using C− V profiling and capacitance transient spectroscopy. The measurements indicate a high density of acceptor-like electron traps about 0.10 eV below the conduction band in a highly localized region in the immediate vicinity of LPE n-In 0.53Ga 0.47As/N-InP isotype heterojunctions. The complete absence of these traps in the interface between two successively grown InP layers, and between two successively grown In 0.53Ga 0.47As layers indicates that the defects are formed during the initial stages of heteroepitaxy. Possible origins of this electron trap are discussed.

Influence of the donor depth on the determination of the band discontinuity of isotype heterojunctions by the capacitance-voltage technique

Capacitance-voltage measurements have been simulated to find the conduction-band discontinuity in an n-type GaAs-Al0.3Ga0.7As heterojunction. In the calculations the partial ionization of the donor level in the (Al,Ga)As is taken into account. It is found that exact knowledge of the donor depth is a prerequisite in order to infer the conduction-band discontinuity from capacitance-voltage measurements. For a dopant concentration of 3×1016 cm−3 on either side of the heterojunction the same apparent carrier profile is obtained with a conduction-band discontinuity equaling 65% of the total band-gap difference and zero donor depth as with 75% discontinuity and 100 meV donor depth. It is also shown that compensation influences the apparent carrier profile. Taking literature values for the donor depth in Al0.3Ga0.7As the discontinuity of the conduction band is estimated to be 70% of the total band-gap difference.

The role of nonuniform dielectric permittivity in the determination of heterojunction band offsets by C–V profiling through isotype heterojunctions

Up to now, C–V profiling through isotype heterojunctions has been performed assuming a uniform dielectric permittivity throughout the heterostructure. We extend the interpretation of C–V data to the case of a semiconductor with position-dependent dielectric permittivity, and we show that the variation of the dielectric permittivity across an isotype heterojunction interface has no effect on the determination of the heterojunction band discontinuity and the interface charge density.

Heterojunction cathode contact transferred-electron oscillators

The purpose of this letter is to clarify the influence of reverse or forward biasing of isotype heterojunction cathode contacts on the nature of the operating mode of millimeter-wave transferred-electron devices. Attractive theoretically expected RF performance are also presented.

Amorphous-crystalline silicon isotype heterojunction: Electrostatic potential distribution and C( V) curves

An amorphous-crystalline silicon isotype heterojunction is studied. The electrostatic potential profile, the built-in and interface potentials, space charge region widths and C- V curves were calculated using different doping levels. The effect of small changes in the electron affinity, band gap and density of states in the gap of a-Si:H were calculated. The effect of a localized charge at the interface was determined. The results were applied to a multijunction of the ppn or nnp type.

Determination of heterojunction band offsets by capacitance-voltage profiling through nonabrupt isotype heterojunctions

It is shown that the capacitance-voltage (C-V) profiling technique to obtain heterojunction (HJ) band offsets is grading independent, that is, the band offset values obtained by it are the values for the limit of zero compositional grading, even in nonabrupt junctions. The result explains the good agreement of old data taken on nonabrupt HJ’s grown by liquid phase epitaxy (LPE) with more recent data on abrupt junctions. It suggests that LPE grown HJ’s may be used more freely than was previously thought to determine those offsets, making C-V profiling the most versatile technique for the determination of HJ band offsets.

Thermionic emission-diffusion theory of isotype heterojunctions

A closed-form expression for the current-voltage characteristics of isotype heterojunctions is developed. Unlike previous treatments of this problem which considered only thermionic emission, this theory, in analogy to metal-semiconductor theory, includes both transport by thermionic emission across the interface and by drift-diffusion in the space charge region. A representative isotype heterojunction is analyzed and it is found that both transport mechanisms must be considered when calculating the current-voltage characteristics.

Electrical properties of ( p)Te-( p)Si isotype heterojunction

The ( p) tellurium-( p) silicon heterojunction has been prepared by vacuum evaporation of tellurium (Te) films on silicon (Si) substrates. The influence of the thickness (1000–13000 Å) and annealing on the electrical properties of the films and heterojunction properties have been investigated. The films prepared on heated substrates (∼425 K) show the properties of the annealed films and good adherance on Si substrates. The current-voltage (I-V) characteristics of the ( p)Te-( p)Si heterojunction have been studied in the temperature range 77 to about 360 K. The I-V characteristics show a double saturation indicating the applicability of double Schottky diode model for isotype heterojunctions when large density of interface states (∼1014) are present. The film thickness and resistivity of the Si (0.4–11 Ω cm) show little influence on the I-V characteristics or builtin potential estimated from the temperature dependence of I-V characteristics. In spite of large difference in band gap of Si and Te the estimated builtin potential on Si (VD1) is only slightly higher than on Te (VD2) side. This suggests that the heterojunction properties are mainly controlled by the interface states. The large interface states in Te-Si heterojunction make the double Schottky diode model valid even when the carrier concentration of Te is more than three orders of magnitude higher than that of Si.

Fabrication of 1-µm Wavelength Region Ag/p-InP/p-InGaAs Schottky Photodiodes by LPE and Selective Etching

Photodiodes capable of operating in the 1-µm wavelength region at room temperature were fabricated by LPE and selective etching techniques. The diodes were formed from a p-InGaAs/p-InP/InGaAsP/InP multiple layer, and they correspond to a series connection, both electrical and optical, of an Ag/p-InP Schottky diode and a p-InP/p-InGaAs isotype heterojunction. It was demonstrated that the photoresponse in the 1-µm wavelength region increases drastically with the applied voltage.

Characterization of GaAs:O–Ge and ZnSe:O–Ge heterointerfaces grown on oxygen-exposed GaAs and ZnSe substrates

The electronic behavior of p–n and isotype heterojunctions is shown to be significantly different when a chemisorbed oxygen/oxide layer is present at the heterointerface. Specifically, the characteristics of nGaAs:O–nGe and nZnSe:O–pGe devices are described. In nGaAs:O–nGe devices we have observed under dark and illuminated conditions a polarity inversion in V–I characteristics not normally expected of conventional nGaAs–nGe heterojunctions. The polarity inversion and other aspects of V–I and C–V characteristics of GaAs:O–Ge devices are consistently explained by a new band model involving the presence of localized inversion and/or accumulation layers at the heterointerface. The novel features in the V–I and C–V characteristics of nZnSe–pGe heterojunctions resulting from the presence of interfacial oxygen are also presented.

Seebeck effect in polycrystalline semiconductors

The paper deals with the interpretation of the Seebeck coefficient measured for a polycrystalline semiconductor. Polycrystalline semiconductors are considered to be composed of grains separated from one another by intergrain domains. An isotype heterojunction with a certain density of interface states is assumed to exist at the grain-intergrain domain interface. The general formula for the Seebeck coefficient under these conditions is derived. The relations valid for systems of practical interest are shown as limiting cases of the formula presented.

P-GaAs/P-Ga1−xAlxAs isotype heterojunctions in doubleheterostructure laser material

The p-GaAs/P-Ga1−xAlxAs isotype heterojunction, that occurs in doubleheterostructure laser material, is studied experimentally and theoretically. In good quality liquid phase epitaxially grown material, carrier-concentration profiles show the presence of hole accumulation in the small band-gap material and hole depletion in the large band-gap material. When either the layer thickness varies across the diode or the heteroboundaries are not abrupt, the profile across the isotype heterojunction becomes smeared. Hence, this profiling method can provide a sensitive measure of material quality. Furthermore, the analysis accounts for the possible presence of interface states, which can affect band bending at heteroboundaries. It is shown that this band bending can affect the recombination of carriers, and therefore their lifetimes and efficiency.

Charge control of the heterojunction two-dimensional electron gas for MESFET application

Previous works have shown that a two-dimensional electron gas (2 DEG) is accumulated at the interface of GaAs (n)/Al x Ga 1-x As (n) isotype heterojunctions. In this paper, a MESFET structure working with this 2 DEG is presented. Theoretical treatments are given, considering that the charge control results from the interpenetration of the Schottky space-charge region and the accumulation layer. A semi-analytical calculation is then developed: conductance, capacitance, source-to-drain current at saturation, and transconductance are predicted for a large-gate FET. Source-to-drain saturation current in short-gate FET is also given. Experimental results obtained in our laboratory and those recently published are compared to calculated data. The good agreement observed in all cases, including low-temperature measurements, clearly shows that the 2 DEG MESFET behavior is actually valid. The high mobility of electrons One can expect from the 2 DEG, particularly at 77 K, suggests that the 2 DEG MESFET is a promising device for microwave and high-speed devices.

Analytic approximations for degenerate accumulation layers in semiconductors, with applications to barrier lowering in isotype heterojunctions

The Joyce-Dixon approximation for the Fermi energy in a degenerate semiconductor is used to obtain rapidly converging series expansions for both the potential and the electric field for degenerate accumulation layers in semiconductors, as functions of the electron concentration. The expressions are valid for electron concentrations up to about 20 times the effective density of states. The approximation is applied to Schottky barriers on degenerate semiconductors, where it leads to an increase in the value of the Gummel-Scharfetter correction in the C-V relation for Schottky barriers. A detailed quantitative treatment is given for the barrier lowering in abrupt n-N heterojunctions with increased reverse bias. Both barrier lowering and reverse bias are given as closed-form parametric expressions, with the electron concentration at the bottom of the heterojunction notch as the independent variable.

Measurement of isotype heterojunction barriers by C-V profiling

The Debye length smearing that occurs in C-V profiling has precluded the use of C-V profiling from an adjacent Schottky barrier to measure the magnitude of energy band discontinuities at barriers in isotype heterojunctions. It is observed, however, that in such a process both the number of the charge carriers and the moment of their distribution are conserved. This information permits the extraction of values for both the conduction band discontinuity ΔEc and any interface charge density. This technique and experimental results for an LPE-grown n-N GaAs-Al0.3Ga0.7As heterojunction are described. We find ΔEc =0.248 eV, corresponding to about to 0.66ΔEg rather than Dingle’s commonly accepted value 0.85ΔEg . The difference is attributed to compositional grading during LPE growth.

Graded Heterojunction Mos and Optoelectronic Devices

In a graded bandgap heterojunction, the variation of bandgap occurs over distances of several diffusion lengths. An examination of the current-transport equation in such devices shows that the electric field resulting from the grading of bandgap and electron affinity can be combined with fields arising from nonuniform donor/acceptor distribution to provide a wide variety of field distributions for electrons and holes. This presentation describes the use of these fields in obtaining improved performance of avalanche devices, solar cells and charge-coupled devices.It is shown that bandgap grading in photodetectors results in smaller recombination and better conversion efficiency. Graded heterojunctions can be used with considerable advantage in imaging applications of charge-coupled devices employing isotype heterojunctions for surface channel and anisotype heterojunctions in bulk channel devices.Experimental study of growth of heterojunctions and their use in MOS device fabrication is briefly described.

Characteristics of isotype n Ge-n GaAs heterojunctions

Isotype heterojunctions make it possible to produce new microwave devices. At present, technological problems are still important and often, there is not a full understanding of their behaviour. In the paper, the fundamental parameters of n Ge - n GaAs heterojunctions are determined from the frequency variations of their impedances Z(ω, V). To account for the large impedance variations, a heterojunction model is proposed. By using the values of the capacitance and the parallel conductance for different frequencies, the fundamental parameters of several isotype heterojunctions are then determined. The calculated characteristics agree well with the experiment.