J J Harris

Optimisation of (Al,Ga)As/GaAs two-dimensional electron gas structures for low carrier densities and ultrahigh mobilities at low temperatures

The authors have grown (Al,Ga)As/GaAs two-dimensional electron gas (2DEG) structures with lightly doped regions of (Al,Ga)As and superlattices in the undoped GaAs. Using this technique, they have obtained ultralow density (2*1010 cm-2), high-mobility samples where phonon scattering at 4 K is the dominant factor in limiting the mobility; reducing the measurement temperature to below 1.5 K gives mobilities of up to 107 cm2 V-1 s-1.

FRACTIONAL QUANTUM HALL EFFECT MEASUREMENTS AT ZERO G FACTOR

Fractional quantum Hall effect energy gaps have been measured as a function of Zeeman energy. The gap at ν = 1/3 decreases as the g factor is reduced by hydrostatic pressure. This behavior is similar to that at ν = 1 and shows that the excitations are spinlike. At small Zeeman energy, the excitation is consistent with the reversal of 3 spins and may be interpreted as a small composite Skyrmion. At 20 kbar, where g has changed sign, the 1/3 gap appears to increase again.

Observation of thermally activated conduction at a GaN–sapphire interface

Temperature-dependent differential Hall measurements have been performed on an undoped GaN epitaxial layer grown by metalorganic chemical vapor deposition on a sapphire substrate. The resultant depth profile shows that, for this sample, the epitaxial material is mostly insulating, and that the observed thermally activated conduction arises from an interface region ⩽0.65 μm thick. Comparison with cross-sectional transmission electron microscope micrographs suggests that this conducting region is correlated to the highly defective three-dimensional growth region, while the two-dimensional growth mode beyond this thickness corresponds to the insulating portion with a lower dislocation density. Such behavior is consistent with the presence of an impurity band in a heavily doped interface region formed by oxygen outdiffusion from the substrate.

The implications of spontaneous polarization effects for carrier transport measurements in GaN

In wurtzite-phase GaN, AlN and InN, the dimensions of the crystallographic unit cell are distorted from the ideal c :a ratio of (8/3)1/2 . This produces a net dipole moment across the cell, with a resultant internal electric field in excess of 1 MV cm-1 , and corresponding polarization charges of ±3-8 × 10-6 C cm-2 on the two surfaces of the epitaxial layer. The effect of this field on the carrier distribution within a film of doped GaN is considered, and shown to produce accumulation and inversion layers of free carriers at opposite surfaces. Theoretical expressions are derived for the effective carrier density and mobility of such films obtained from Hall measurements, and compared with characteristic experimental results. Qualitatively consistent behaviour is observed in some, but not all, samples, but quantitative agreement is generally lacking, and possible explanations for this are considered.

Detailed interpretation of electron transport in n-GaN

We report on temperature-dependent differential Hall-effect and resistivity measurements, between 10 and 300 K, on two silicon doped GaN epitaxial layers grown by two different metalorganic chemical vapor deposition sources on sapphire substrates. Reactive ion etching has been used to enable Hall measurements to be taken as a function of film thickness, for 1.62 and 3.92 μm thick films. Temperature-dependent Hall experiments indicate classical donor freeze-out in the doped region, while the depth profile measurements show that in the undoped layer, the Hall electron density passes through a minimum before increasing again at lower temperatures. Such behavior is indicative of impurity conduction in this region. Using a model based upon a doped layer with one type of shallow donor, plus compensating acceptors, in parallel with an interface layer which shows impurity-band conduction, a simultaneous fitting of mobility and carrier concentration has been undertaken to quantify the contribution of different scattering mechanisms, and the densities of the donors and acceptors in the doped region were also found. An important result is that for the carrier concentration fitting, when the temperature dependence of the activation energy is taken into account, the fitted energies are closer to those predicted by free and bound carrier screening. Finally the Fermi level was found to be in the range ∼56–84 meV below the conduction band minimum at room temperature and moved closer to the donor levels at low temperatures.

Mobility collapse in undoped and Si-doped GaN grown by LP-MOVPE

The room temperature carrier mobility in bulk GaN layers is found to improve drastically by up to a factor of 20 once the incorporated silicon is higher than a critical value. A theoretical model taking into account several scattering mechanisms has been developed to account for the temperature dependence of conduction band mobility m(T) and carrier density n(T) deduced from Van der Pauw Hall measurements. For significantly Si-doped layers, both m(T) and n(T) can be quite accurately reproduced. In lightly doped and undoped samples, m(T) cannot be explained just using this model. TEM observation shows the presence of a more diffuse distribution of threading dislocations in the Si-doped material compared to the undoped one. The grain boundaries in this latter case are likely to correspond to high energetic barriers that carriers can overcome only by some tunneling process resulting in very low mobility.

Relationship between classical and quantum lifetimes in AlGaN/GaN heterostructures

Carrier transport in a set of AlGaN/GaN heterostructures from different sources with a range of carrier densities and mobilities has been investigated at low temperature and high magnetic fields. The Shubnikov-de Haas oscillations have been analysed to extract the quantum scattering lifetime, τq, and this is compared with the classical transport lifetime, τt, derived from the low-field mobility. The relationship between these parameters has been observed to depend systematically on the low-field mobility of the samples studied, and indicates that higher-mobility samples suffer less scattering from centres close to the two-dimensional conducting channel.

Investigation of phonon emission processes in an AlGaN/GaN heterostructure at low temperatures

We have measured the temperature- and field-dependent magnetoresistance in an undoped AlGaN/GaN two-dimensional electron gas sample at temperatures below 10 K. Well-resolved Shubnikov-de Haas oscillations have been observed, as well as spin splitting at magnetic fields above 8 T. The amplitude of the oscillations has been used as a thermometer for the electron temperature, Te, studies as a function of driving current have shown that under high applied electric fields the power input per electron follows a Te4.4 dependence. Comparison with numerical modeling indicates that in this heterostructure, electron-acoustic phonon scattering via the screened piezoelectric interaction is the dominant energy-loss mechanism at low temperatures.

Interpretation of the Temperature‐Dependent Transport Properties of GaN/Sapphire Films Grown by MBE and MOCVD

Electron transport in Si-doped and unintentionally-doped GaN films grown on sapphire by MOCVD and MBE has been analysed assuming that a parallel conducting channel, via an impurity band, is present. No dependence on growth method or dopant type was observed, but other trends were apparent: a) the activation energy for the impurity band fell with increased doping; b) the temperature of the minimum in the Hall carrier density versus temperature curves increased with doping, but did not depend strongly on the absolute value of mobility; c) the ratio of the mobility in the GaN conduction band to that in the impurity band also showed systematic behaviour, possibly arising from structure-related scattering processes. An STM study of the surface characteristics of some of these samples suggests that potential variations associated with particular structural features may be important in influencing the electrical properties.

Multiple parallel conduction paths observed in depth-profiled n-GaN epilayers

We have combined plasma etching with the Hall effect and resistivity measurements between 10 and 300 K to study the depth distribution of conduction in silicon (Si)-doped GaN epitaxial layers grown on sapphire substrates by two different metalorganic chemical vapor deposition processes. Reduction of the epitaxial layer thickness produces a linear decrease of the sheet carrier density with depth in the doped region, whilst in one sample, in the region less than ∼0.3 μm from the interface, the sheet carrier density tends to flatten out to a value of ∼3×1013 cm−2. The former is indicative of a uniform dopant distribution in the epitaxial material, and the latter reveals the existence of mobile charge near the interface. These experiments allow the properties of the doped material to be deconvoluted from those of the interface region, and the temperature dependence of these properties indicates the presence of two parallel conduction paths in the doped material: the conduction band and an impurity band. Thus ...