Chuanle Zhou

Driving perpendicular heat flow: (p×n)-type transverse thermoelectrics for microscale and cryogenic peltier cooling

Whereas thermoelectric performance is normally limited by the figure of merit ZT, transverse thermoelectrics can achieve arbitrarily large temperature differences in a single leg even with inferior ZT by being geometrically tapered. We introduce a band-engineered transverse thermoelectric with p-type Seebeck in one direction and n-type orthogonal, resulting in off-diagonal terms that drive heat flow transverse to electrical current. Such materials are advantageous for microscale devices and cryogenic temperatures--exactly the regimes where standard longitudinal thermoelectrics fail. InAs/GaSb type II superlattices are shown to have the appropriate band structure for use as a transverse thermoelectric.

Thermal Sensing With Lithographically Patterned Bimetallic Thin-Film Thermocouples

A chromium-nickel thin-film thermocouple that is 50 nm thick is demonstrated on a semiconductor substrate as proof of concept for lithographically processed bimetallic on-chip temperature sensors. The Seebeck coefficient of the thin-film thermocouple is calibrated to be 10.37 μV/°C, reproducibly smaller than the bulk literature value by a factor of 3.98. The batch reproducibility of this thin-film Seebeck coefficient is demonstrated. The linear Seebeck response up to 90°C is calibrated with the help of a simple formula which accounts for the temperature variations of the reference thermocouple under large heat loads.

DC reactive magnetron sputtering, annealing, and characterization of CuAlO2 thin films

CuAlOx thin films were prepared at three substrate temperatures (TS=60, 300, and 600 °C) and two oxygen partial pressures (PO2=0.5 and 2 mTorr) via dc reactive magnetron sputtering from Cu–Al 50–50 at. % alloy targets and subsequent annealing. As-deposited films with PO2=0.5 mTorr were oxygen deficient; although the delafossite structure formed upon annealing, electrical properties were poor. Films deposited with PO2=2 mTorr transformed into the delafossite structure and exhibited p-type conductivity after annealing under N2 at temperatures TA≥750 °C. Conductivity generally increased with increasing TS and decreasing TA. A special case of PO2=2 mTorr and low TS (60 °C) resulted in a partially crystalline oxide phase that transformed into the delafossite structure at TA=700 °C and yielded the highest conductivity of 1.8 S cm−1. In general, a TA near the phase formation boundary led to an increase in conductivity. Low-temperature hydrothermal annealing was also investigated and shown to produce mixed phase ...

Thermal conductivity tensors of the cladding and active layers of interband cascade lasers

The cross-plane and in-plane thermal conductivities of the W-active stages and InAs/AlSb superlattice optical cladding layer of an interband cascade laser (ICL) were characterized for temperatures ranging from 15 K to 324 K. The in-plane thermal conductivity of the active layer is somewhat larger than the cross-plane value at temperatures above about 30 K, while the thermal conductivity tensor becomes nearly isotropic at the lowest temperatures studied. These results will improve ICL performance simulations and guide the optimization of thermal management.

Thermal conductivity tensors of the cladding and active layers of antimonide infrared lasers and detectors

The in-plane and cross-plane thermal conductivities of the cladding layers and active quantum wells of interband cascade lasers and type-II superlattice infrared detector are measured by the 2-wire 3ω method. The layers investigated include InAs/AlSb superlattice cladding layers, InAs/GaInSb/InAs/AlSb W-active quantum wells, an InAs/GaSb superlattice absorber, an InAs/GaSb/AlSb M-structure, and an AlAsSb digital alloy. The in-plane thermal conductivity of the InAs/AlSb superlattice is 4–5 times higher than the cross-plane value. The isotropic thermal conductivity of the AlAsSb digital alloy matches a theoretical expectation, but it is one order of magnitude lower than the only previously-reported experimental value.

Generalized four-point characterization method using capacitive and ohmic contacts

In this paper, a four-point characterization method is developed for samples that have either capacitive or ohmic contacts. When capacitive contacts are used, capacitive current- and voltage-dividers result in a capacitive scaling factor not present in four-point measurements with only ohmic contacts. From a circuit equivalent of the complete measurement system, one can determine both the measurement frequency band and capacitive scaling factor for various four-point characterization configurations. This technique is first demonstrated with a discrete element four-point test device and then with a capacitively and ohmically contacted Hall bar sample over a wide frequency range (1 Hz-100 kHz) using lock-in measurement techniques. In all the cases, data fit well to a circuit simulation of the entire measurement system, and best results are achieved with large area capacitive contacts and a high input-impedance preamplifier stage. An undesirable asymmetry offset in the measurement signal is described which can arise due to asymmetric voltage contacts.

Thermal conductivity tensor of semiconductor layers using two-wire 3-omega method

We used the two-wire 3ω method to measure the in-plane and out-of-plane thermal conductivity of thin films and analyzed the error for all fitting parameters. We find the heater half-width, the insulating layer thickness and the out-of-plane thermal conductivity of the insulating layer the most sensitive parameters in an accurate fitting. The data of a 2.5 μm GaAs thin film suggests that the phonon mean free path in the film is limited to the film thickness, far shorter than that in the bulk material at low temperatures.

Thermal distribution in high power optical devices with power-law thermal conductivity

We introduce a power-law approximation to model non-linear ranges of the thermal conductivity, and under this approximation derive a simple analytical expression for calculating the temperature profile in high power quantum cascade lasers and light emitting diodes. The thermal conductivity of a type II InAs/GaSb superlattice (T2SL) is used as an example, having negative or positive power-law exponents depending on the thermal range of interest. The result is an increase or decrease in the temperature, respectively, relative to the uniform thermal conductivity assumption.

p × n-type transverse thermoelectrics: an alternative Peltier refrigerator with cryogenic promise

This work describes a band-engineered transverse thermoelectric with p-type Seebeck in one direction and ntype orthogonal, with off-diagonal terms that drive heat flow transverse to electrical current. Such materials are named p × n type transverse thermoelectrics. Whereas thermoelectric performance is normally limited by the figure of merit ZT, p × n type materials can be more easily geometrically shaped and integrated for devices, leading to more compact, longer lifetime, enhanced efficiency coolers for infrared detectors or photovoltaic generators.

InAs/GaSb Type II Superlattices as Low- Temperature Thermoelectrics

Leo Esaki originally proposed that by increasing the layer thickness, InAs/GaSb superlattices can be tuned from a semiconducting to a semimetallic state, where electron and hole wavefunctions are spatially localized to InAs and GaSb layers, respectively. Because of the tunably small spatially indirect gap of InAs/GaSb and the anisotropy of the superlattice structure, this material might have interesting thermoelectric applications at cryogenic temperatures. We measured the thermal conductivity of such Type II superlattices using the 3ω method and observe a reduction by two orders of magnitude from the average GaSb and InAs bulk thermal conductivities. We also use 8×8 band k⋅p envelope‐function approach to simulate the dispersion function of different period InAs/GaSb superlattices, and we find the InAs and GaSb layer thicknesses can be adjusted to engineer anisotropic band structures.