Giant electric fields in unstrained GaN single quantum wells

  • Langer R
  • Simon J
  • Ortiz V
 et al. 
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Abstract

We demonstrate that, even in unstrained GaN quantum wells with AlGaN barriers, there exist giant electric fields as high as 1.5 MV/cm. These fields, resulting from the interplay of the piezoelectric and spontaneous polarizations in the well and barrier layers due to Fermi level alignment, induce large redshifts of the photoluminescence energy position and dramatically increase the carrier lifetime as the quantum well thickness increases. © 1999 American Institute of Physics. ͓S0003-6951͑99͒04225-4͔ In biaxially strained nitride heterolayers grown in the wurtzite structure with the c axis parallel to the growth di-rection, giant piezoelectric and spontaneous polarization ef-fects are expected to be present as a consequence of the noncentrosymmetry of the wurtzite structure. 1 Actually, for fully strained GaN on AlN (⑀ϭ2.4%) piezoelectric fields as high as several MV/cm are expected. These values are more than an order of magnitude larger than the piezoelectric fields that can be found in zinc blende semiconductors for the same amount of strain. 2 Nevertheless, the role of these giant piezoelectric fields on the optical properties of strained nitride nanostructures has only recently started to be as-sessed. For example, large piezoelectric field-induced red-shifts in photoluminescence ͑PL͒ spectra of strained GaN/ AlGaN quantum wells ͑QWs͒, 3 InGaN/GaN QWs, 4 and GaN/AlN self-assembled quantum dots 5 have been reported lately. In this work, we show that strong electric fields are present even in unstrained GaN QWs 6 with strained AlGaN barriers, and that the origin of these fields is the outcome of balancing the piezoelectric and spontaneous polarizations in the well and barrier layers in order to achieve a flat Fermi level throughout the heterostructure. The multiple quantum well ͑MQW͒ sample discussed here consists of three single GaN QWs of 10, 20 and 30 Å nominal thickness, separated by 50 Å Al 0.24 Ga 0.76 N barriers. The total AlGaN thickness is 200 Å and is fully strained, as shown below by high resolution x-ray diffraction. The sample is grown at 650 °C by molecular beam epitaxy using a rf-plasma nitrogen source. Details of the growth procedure are given elsewhere. 6 As a substrate, we used a commercially available 1.5-␮m-thick GaN layer grown by metalorganic chemical vapor deposition on sapphire. Between the sub-strate and the MQW section we interposed 100 Å of GaN buffer. The time integrated and time resolved PL experi-ments presented here were performed with a standard streak camera setup at Tϭ10 K. The excitation was provided by 265 nm pulses of 1.5 ps duration at 76 MHz repetition rate. Typical average power densities used in these measurements were 50 W/cm 2 . It is essential to any work dealing with piezoelectricity in a strained system, such as the GaN/AlGaN one, to know accurately the strain configuration of the heterostructure. A good method for this is x-ray measurement of reciprocal lat-tice map around asymmetrical Bragg peaks. In Fig. 1, we show the reciprocal lattice map of the MQW sample around the ͑202͒ Bragg peak of the GaN substrate. Aside from the substrate peak, we distinguish an additional peak due to the Al 0.24 Ga 0.76 N barrier layers. The first important information that can be drawn from the map is that the alloy has the same in-plane lattice parameter ͑represented by the H axis͒ as the GaN substrate. This means that the AlGaN of this thickness and composition is fully strained to GaN, without any onset of relaxation. Hence, we can conclude that the MQW hetero-structure is pseudomorphic on the GaN substrate, i.e., the GaN QWs are unstrained. From the in-plane lattice

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