P. Hendriks

Spatial potential distribution in GaAs/AlxGa1−xAs heterostructures under quantum Hall conditions studied with the linear electro-optic effect

Abstract We apply the linear electro-optic effect (Pockels effect) to investigate the spatial potential distribution in GaAs/AlxGa1−xAs heterostructures under quantum Hall conditions. With this method, which avoids electrical contacts and thus does not disturb the potential distribution, we probe the electrostatic potential of the two-dimensional electron gas locally. Scanning across the width of the sample inside a quantized Hall plateau we observe a steep change of the Hall potential at the edges of the two-dimensional electron gas. This steep change occurs over a distance of about 70 microm, which is the lateral resolution of the experimental set-up. More than 80% of the total Hall voltage is concentrated near the edges. The remainder of the Hall potential is distributed in the interior of the sample and varies linearly with the position. The results are interpreted in terms of unscreened charge at the edges. If the plateau region is left or if the quantized Hall conditions are violated by increasing the temperature or current level the Hall potential becomes a linear function of position.

Electric field induced parallel conduction in GaAs/AlGaAs heterostructures

A new mechanism to understand time‐dependent features in the conduction of a two‐dimensional electron gas (2 DEG) in high electric fields is proposed and discussed. The mechanism is based on the idea that not only the properties of the GaAs/AlGaAs heterostructure have to be included, but also the properties of the transition from the ohmic contact to the heterostructure. We show that the ohmic contact to the heterostructure is fundamentally different from the contact to a bulk semiconductor. In low electric fields electrons cannot move from the contact into the AlGaAs since the conduction band normally lies above the Fermi level. However, when high enough electric fields are applied, the barrier between the contact and AlGaAs is pulled down, allowing conduction in both the AlGaAs and the 2 DEG. We propose a model, in which time‐dependent phenomena in the conduction of the 2 DEG can be associated with trapping and detrapping of charge carriers in the AlGaAs. Time‐resolved experiments are shown which confir...

Use of the lateral photoeffect to study sample quality in GaAs/AlGaAs heterostructures

We have used the lateral photoeffect to image the variations in the conductivity of a two‐dimensional electron gas (2DEG) setup in a GaAs/AlGaAs heterostructure. A description of the experimental arrangement is given and a simplified theory of the image contrast is presented. Our experiments image contrast resulting from defects such as cracks in the GaAs sublayer, and other contrast resulting from local changes in resistance of the 2DEG. Such variations have important consequences concerning both the interpretation of average parameters measured on such 2DEGs and the performance of electronic devices ( such as high electron mobility transistors ) fabricated using them. The inhomogeneities no doubt have their origin either in defects in the substrate used, or in the growth of the layers, or both.

Contactless measurement of the spatial potential distribution in the quantum hall effect

Abstract For the first time a contactless method based on the Pockels effect is used to determine the spatial potential distribution in the two-dimensional electron gas of a GaAs/Al x Ga 1− x As heterostructure under quantum Hall conditions. From first results we deduce that in the two-dimensional electron gas structure edge currents as well as currents in the interior are present.

Origin of current instabilities in GaAs/AlxGa1-xAs heterostructures : avalanche ionization in the AlxGa1-xAs layer

Pulsed current‐voltage measurements on modulation‐doped GaAs/AlxGa1−xAs heterostructures are presented at electric fields up to 2 kV/cm. At fields between 0.5 and 2.0 kV/cm we observe up to three well‐defined avalanche type current jumps as a function of time. These current jumps show hysteresis effects as a function of the electric field. At even higher electric fields the current becomes irregular and we observe chaotic behavior. To explain the current instabilities we assume that at high electric fields electrons are injected into the AlxGa1−xAs layer parallel to the two‐dimensional electron gas. The injected electrons subsequently cause avalanche ionization of occupied DX centers in the AlxGa1−xAs layer. Due to this process, a current filament is created with an exceptionally high mobility which is about 2×104 cm2/V s at 10 K.

Observation of current filaments in GaAs/AlxGa1−xAs heterostructures using a time‐resolved imaging technique

We developed a time‐resolved optical beam induced current (TROBIC) technique, and performed time‐resolved current imaging experiments on GaAs/AlxGa1−xAs heterostructures under high electric field conditions. These experiments are the first time‐resolved imaging experiments of current patterns in a two‐dimensional semiconductor structure. We attribute the current patterns observed in the TROBIC images to the formation of current filaments in the AlxGa1−xAs layer, parallel to the two‐dimensional electron gas (2DEG). We show that even in samples where the two‐dimensional electron gas and the contacts to the 2DEG are perfectly ohmic and homogeneous, current filaments can still develop in high electric fields. These temporal and spatial instabilities in the AlxGa1−xAs layer strongly affect the high‐field transport properties of the heterostructure.

Differential measurements of the lateral photoeffect in GaAs/AlGaAs heterostructures

The authors present a novel laserscan technique based on the lateral photoeffect to image the local resistivity of the two-dimensional electron gas of GaAs/AlGaAs heterostructures. They show that a measurement of the spatial derivative of the lateral photoeffect gives direct information about the local resistivity. Compared with conventional measurements of the lateral photoeffect the technique presented here is able to resolve much smaller variations in the resistivity. This is illustrated by the experimental results presented.

Electro‐optic voltage profiling of modulation‐doped GaAs/AlGaAs heterostructures

The electro‐optic effect of GaAs is applied to profile the voltage distribution of the two‐dimensional electron gas (2DEG) in a GaAs/AlGaAs heterostructure. In our setup we reached a voltage sensitivity of 2 mV. We used this technique to characterize the local resistivity of the 2DEG. The results are consistent with those obtained from scanning electron microscopy voltage contrast measurements.

Modulation doping and delta doping of III-V compound semiconductors

The transport properties of the 2D electron gas produced by modulation doping of compound semiconductors are reviewed with attention given to the properties at high electric fields. Experimental studies are discussed in which the transport properties lead to insights into current instabilities and switching effects. The concept of electric-field-induced parallel conduction is set forth and shown to explain the current instabilities and current collapse at high electric fields. Delta doping is shown to be effective for electrooptic devices such as modulators. MQW modulators with delta-doped contacts can be used as waveguides in complicated coupler networks, or they can be optimized for a high on/off ratio by increasing device length without increasing propagation loss.

Current oscillations at high electrical fields in the two‐dimensional electron gas in GaAs/AlGaAs heterostructures

In the two‐dimensional electron gas (2DEG) of GaAs/AlGaAs heterostructures, we observe a new type of oscillation in the current at electric fields above 0.8 kV/cm. These oscillations have two characteristic frequencies, around 200 MHz and 2.5 GHz. We show that these oscillations arise from properties of the 2DEG and depend on both electric field and magnetic fields. They disappear abruptly when magnetic fields above 0.5 T are applied.