Gottfried H. Dohler

Doping superlattices ("n-i-p-i crystals")

Semiconductors with doing superlattices exhibit a number of unique features by which they differ from uniform bulk crystals as well as from semiconductors with a compositional superlattice. The properties which make them particularly appealing as a new kind of semiconductor are tunability of carrier concentration, bandgap, two-dimensional subband structure, and recombination lifetimes, in combination with an enormous flexibility in tailoring. Moreover, the choice of host materials is not restricted by interface- or lattice-matching problems. The possibility of varying conductivity, absorption coefficient, optical gain, and luminescence spectra by light or external electrical potentials implies new concepts for photodetectors, tunable light sources, optical amplifiers, and modulators. The long recombination lifetimes result in large low-power nonlinearities of the optical absorption coefficient and the refractive index. These properties offer applications for saturable absorbers and bistable optical devices. In this paper the basic concept of doping superlattices and experiments which have provided its verification will be reviewed first. New physical phenomena and possible device applications will then be discussed. Finally, we will report some recent results of extensions of the concept to hetero n-i-p-is, obtained by periodic modulation of composition superimposed on the periodic n-i-p-i doping profile.

In situ grown‐in selective contacts to n‐i‐p‐i doping superlattice crystals using molecular beam epitaxial growth through a shadow mask

Highly selective n‐ and p‐type contacts to GaAs doping superlattices have been achieved by using molecular beam epitaxial growth through a silicon shadow mask to form interdigital grown‐in contacts. Low contact resistance, excellent diode characteristics, and efficient lateral injection electroluminescence are obtained.

Investigation of luminescence and non-linear optical properties of hetero n-i-p-i superlattices

Abstract Hetero n-i-p-i crystals are periodic arrays of n- and p-doped layers with interspersed undoped (i-) layers of a different, smaller band gap semiconductor. They combine the interesting and useful tunability of conventional doping superlattices (“n-i-p-i crystals”) with the only weakly broadened subhand structure of modulation-doped hetero-structure superlattices. Two different versions of such superlattices will be discussed. The first temperature-, intensity-, and time-dependent luminescence measurements on “type-II hetero n-i-p-is” confirm the expected fine structure of the luminescence spectra. New non-linear optical phenomena like absorption oversaturation are predicted for “type-I hetero n-i-p-is”.

The potential of n-i-p-i doping superlattices for novel semiconductor devices

Abstract In this paper we suggest a number of device applications of n-i-p-i doping superlattices. The concept of these devices is based on the unusual electronic properties of this new class of semiconductors such as extremely long excess carrier lifetime, tunable band gap and carrier concentration. Emphasis will be on high-sensitivity low-noise photodetectors, tunable lasers, optical amplifiers, and on ultrafast devices for the generation, modulation and detection of optical signals.

Theory of amorphous multilayer structures (superlattices)

Abstract A comparison is made between compositional and doping superlattices in crystalline semiconductors and their amorphous counterparts. We show that peculiarities similar to those of crystalline superlattices can be observed also in amorphous multilayer structures. The study of these new materials provides numerous possibilities to obtain information about the properties of amorphous bult materials.

High-speed n-i-p-i Photodetector with internal gain**

A new type of infrared detector made from n-i-p-i doping superlattice crystals with laterally graded doping is proposed. Photogenerated electrons are rapidly and efficiently swept by a built-in drift field into a small area for detection with high pholoconductive gain. The operation at voltages much below 1 volt implies very low noise. Inspite of the low voltage the capacitance is also much smaller than in conventional p-i-n detectors. The sensitivity of optical receivers using this detector may become photon-noise limited in the low Gbit/s range. Practical advantages of the device include modest requirements on crystal purity and on control of doping level and its suitability for monolithic integration.

Light Generation, Modulation, And Amplification By N-I-P-I Doping Superlattices

The n-i-p-i doping superlattices, periodic structures composed of n- and p-doped layers, possibly with undoped (i-) regions of the same semi-conductor material in between, form a new class of semiconductors. Apart from their two-dimensional and highly anisotropic electronic structure, a property they share with their compositional superlattice counterparts, they exhibit widely tunable electrical and optical properties that make them very appealing for various applications. Carrier concentration, lumines-cence spectra, absorption coefficient, and refractive index can be modulated by unusually large amounts by weak light signals, injection currents, or external fields. We briefly review the basic theoretical concept and some of the experiments that have confirmed the theory. Then we describe a number of optoelectronic and purely optical device applications. These include wavelength-tunable incoherent and coherent light-emitting devices with high modulation frequency, fast optical modulators, and nonlinear optical devices, possibly exhibiting optical multistability. Although the concept is applicable to any semiconductor that can be doped appropriately, a combination of periodic doping and composition (hetero n-i-p-i) exhibits particularly appealing potential for devices in the 0.8 to 1.55 µm wavelength range.

On the Determination of the Gap Density of States in Amorphous Semiconductors from Investigations on Doping Superstructures

A new approach for determining the gap density of states in amorphous semiconductors is discussed. It is expected that the results of this method are neither influenced by any surface or interface effect. Also the method has the advantage that measurements are made in thermal equilibrium.

Tunable absorption and electroluminescence in GaAs doping superlattices

Abstract Highly tunable electroluminescence is observed in GaAs doping superlattice (n-i-p-i crystal) at room temperature with peak energies shifted more than 600 meV below the bulk bandgap (λ > 1.55 μm). Peak efficiency is about 2 %. Tunability of the optical absorption spectrum with p-n junction bias is also demonstrated by both photoconductivity and direct transmission measurements. A change of transmission of about 9% is obtained at 0.89 μm wavelength through a 1.95 μm thick n-i-p-i crystal by varying the p-n junction bias between −0.5 V and 0.5 V.

Temperature dependence of the tunable luminescence of GaAs doping superlattices

Abstract Doping superlattices show tunable optical and electrical properties due to the space charge induced separation of photoexcited or electrically injected carriers. We have investigated the tunable luminescence in GaAs doping superlattices of doping levels n=1×1018cm−3 and n=4×10 18cm−3 as function of excitation density and sample temperature. The temperature dependence of the tunability was investigated in the range between 4 and 700K, and we found the critical transition temperatures T0 at 90 and 460K for the low and high doped samples, respectively. The results verify the theoretical prediction concerning the transition temperature at which the luminescence changes from full to zero tunability.