ELECTRICAL CHARACTERIZATION OF LOW-TEMPERATURE-GROWN GaAs

Author:  James Ibbetson

[Questions regarding this research may be addressed to]: ibbo@indy.ece.ucsb.edu

We have been studying the electrical properties of low-temperature-grown (LTG) GaAs. The goal has been to identify and tailor the microscopic properties of LTG GaAs (i.e. defects and traps) that are responsible for its useful macroscopic electrical properties such as high resistivity and high breakdown fields.

Low field resistivity measurements on bulk films

A simple way to characterize the material is by measuring the resistivity using a current-voltage (I-V) measurement of a metal-i-n+ structure, with the i layer being ~1 µm LTG GaAs. One of the most interesting aspects of LTG GaAs is that its properties are very sensitive to the growth temperature and also to the temperature of a postgrowth anneal, as shown in Figure 1. In brief, the resistivity
changes are correlated to changes in the density of excess-As related defects in the material: the lower the growth temperature the
higher the density; the higher the anneal temperature the lower the density of defects. The resistivity of >10⁷ ohm-cm of material annealed at 600°C or above makes LTG GaAs ideal for device applications in which an insulator or current blocking layer is required.

Figure 1: (a) Effect of growth temperature (open circles) on the resistivity of LTG GaAs films and the effect of a 30 second anneal at 600°C. (b) Evolution of resistivity with anneal temperature in LTG GaAs grown at 350°C and 250°C.

Temperature-dependent I-V measurements reveal that the type of defect that dominates or controls the electrical properties also changes with annealing. As shown in Figure 2, the energy signature of the trap that pins the Fermi level position changes from 0.4 eV in as-grown material, to 0.7 eV in material annealed at 600°C.

Figure 2: Arrhenius plot of the low field conductivity in LTG GaAs grown at 350°C and annealed at various temperatures.

Direct measurement of the trap density of states

As the data above suggest, the density of states (DOS) spectrum in the band gap of LTG GaAs is not as simple as a single deep level or even a partially compensated deep level. For material annealed at 500°C or above this is not too surprising, perhaps, given the presence of As precipitates which form as the excess As incorporated at low temperature anneals out and coalesces. It has been shown that the precipitates are crystalline As of either a semi-metallic or metallic phase depending on their size. In either case, it is reasonable to expect the precipitates to contribute states that line up in the bandgap of the surrounding GaAs matrix. Indeed, this led to the proposal that the precipitates act like buried Schottky barriers and that they are directly responsible for (among other things) the semi-insulating nature of annealed LTG GaAs.

Figure 3: Bright field TEM images of two samples from the same LTG GaAs epilayer, grown at 225°C and annealed for 30 seconds at (a) 600°C, and (b) 750°C. The dark 'spheres' are As precipitates. The average precipitate diameter is 50 Å and 115 Å in (a) and (b), respectively.

In order to investigate the role of precipitates versus that of point defects, we have performed an experiment designed to measure the spectrum of gap states in LTG GaAs. The basic idea involves sandwiching a thin LTG GaAs layer between a lightly doped channel and an undoped, wide bandgap (Al_0.45Ga_0.55As) cap. By applying a bias between an Ohmic contact to the channel and a Schottky contact to the cap, the traps in the LTG GaAs can be emptied and filled in a controllable way.

Figure 4: Schematic conduction band diagram and space charge distribution for a structure containing a thin LTG GaAs layer inserted between an AlGaAs current blocking cap and a n-GaAs channel. Trap states initially above the Fermi level are filled under forward bias; those initially below it are emptied under reverse bias.

More importantly, the total charge (and thereby the DOS) in the LTG GaAs can be determined from the bias-dependent capacitance of the structure, which is an easily measured quantity. Results for a 100Å thick LTG GaAs layer grown at 250°C and annealed at 600°C are shown in Figure 5. Two main features are observed in the DOS: (1) a narrow peak at 0.6-0.65 eV below the conduction band edge; (2) a ~200 meV broad band of states centered near 0.4 eV below the conduction band edge. The charge neutral Fermi level is pinned in the narrow peak of states.

Figure 5: Net charge in a 100Å thick LTG GaAs layer as a function of the Fermi level determined from room temperature C-V measurement performed on a structure like the one in Figure 3. The trap density of states is just the derivative of the charge.

By studying the DOS in different samples as a function of growth and anneal temperature, and also by comparing the density of traps with the bulk conductivity results, we have been able to conclude that these features are point defect-related. Thus the precipitates do not appear to be electrically important, at least in the material studied to date. Efforts are currently under way to increase the density of precipitates in order to "see" their electronic signature.

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