Índices generales de calidad del agua

A com bination of Hall and low tem p e ra tu re m a g n e to -tra n sp o rt m easurem ents has been successfully im plem ented in this thesis to investigate electrical p ro p erties of n- ty p e G aN layers and A lG aN /G aN 2DEG system s. G roup III n itrides contain a high level of disorder arising m ainly from non-optim ised grow th conditions, and from tra n s ­ p o rt studies of th e samples, inform ation on th e role of s tru c tu ra l defects and th eir effects regarding device application is obtained. In this chapter, an overall su m m ary of th e m ain results discussed in this thesis is presented, followed by a list of suggestions for fu tu re work.

7.1

Overall sum m ary

T h e tem p e ra tu re -d ep e n d e n t Hall effect in n-G aN sam ples grown on sapphire by M B E and M O C V D has been analysed using a m odel w ith and w ith o u t an im p u rity band. T ra n sp o rt properties of non-degenerate G aN layers unaffected by an im p u rity b an d conduction have been investigated by sim ultaneously fitting th e H all m obility and co n centration d a ta , and found th a t (a) a t high tem p e ra tu re s (> 200 K ), p olar optical sc atterin g dom inates th e mobility, w hereas a t lower tem p e ra tu re s, ionised im p u rity sc atterin g is th e dom inant m obility lim iting m echanism , (b) th e Na values e x tra c te d from th e low te m p e ra tu re m obility fit indicate com pensation in th e range 20 — 40%, w here a higher m obility is associated w ith th e less com pensated sam ple, (c) th e ob­ tain e d donor activation energy 10 meV) agreed closely w ith th e th eo retical q u a n ti­

7. C o n clu sio n s 160

ties pred icted for th e hydrogenic donors w ith a screening effect.

For th e sam ples w ith doping densities in excess of 1 x 10^® cm “ ^ (M ott criterion), th e presence of a parallel conduction channel (im purity band) and its effect on elec­ tro n tra n s p o rt are studied by de-convoluting th e te m p e ra tu re -d ep e n d e n t H all d a ta . T he de-convoluted results indicated th a t (a) th e electrical p ro perties are not system ­ atically dependent on grow th technique, (b) Si and u n in ten tio n al donors show sim ilar behaviour, (c) th e im purity band is approxim ately (30 ± 9) m eV below th e conduction b a n d edge a t low im p u rity densities, b u t th is energy reduces w ith increasing im p u rity content, (d) th e m obility degradation m echanism s have th e sam e relative effect on elec­ tro n tra n s p o rt in th e conduction and im p u rity bands, and (e) stru c tu ra l defects such as grain boundaries are associated w ith p o ten tia l fluctuations which could be p a rtially responsible for reducing mobility.

T ra n sp o rt properties of A lG aN /G aN h e te ro stru ctu res grown on sapphire by M B E and M O C V D have also been studied, based on in te rp re ta tio n s of th e H all and low tem ­ p e ra tu re m ag n eto -tran sp o rt m easurem ents (m ainly utilising th e SdH effect). T h e pres­ ence of a 2DEG is confirmed by th e clear SdH oscillations which have been seen in th e wide range of sam ples characterised, covering th e m obility from a b o u t 1000 to 28 000 cm^ s~^. T he high 2D sheet electron densities in th e range (2.6 — 10.6) x 10^^ cm~^ have been m ainly a ttrib u te d to th e charges induced by spontaneous and piezoelectric po larisatio n effects, which become stronger w ith th e increasing A1 com position and b a r­ rier thickness. D eviations from th e ideal pxx variation w ith B have been observed in some of th e sam ples, arising from th e parallel conduction effects and electron-electron interactions. However, th e close agreem ent betw een th e carrier density values ob tain ed from th e SdH m agnetoresistance and H all resistance suggested th a t these effects are m inim al. T h e effective m ass values calculated from th e SdH oscillation am plitudes are found to be 0.22mo and 0.25mo for th e sam ples w ith high and low m ate ria l quality, respectively, and w ithin th e previously rep o rted range.

A fairly consistent underlying tre n d in th e scatterin g behaviour, in d ep en d en t of grow th technique, is observed by studying th e relationship betw een Tq and Tf. For a given sc atterin g rate, th e results show th a t th ere is a g reater p ro p o rtio n of large-angle sc atterin g in n itrid e sam ples (1 ^ Tt/rq ^ 12) th a n in G aA s/A lG aA s h e tero ju n ctio n s

7. C o n clu sio n s 161

{jtjTq >> 10). T h e sm aller Dingle ratio values probably occur from th e presence of dislocations, higher background im purity densities and m ore severe interface roughness. T h e large variation of m obility exhibited w ithin one sam ple (up to 3-fold) corresponds to inhom ogeneities in th e layer, and this evidence seems to point to grain b o u n d a ry sc atterin g as th e dom inant m obility lim iting process. T he p ro perties of two sam ples containing deliberate m odulation doping display g reater short-range scattering, p a rtly due to th e increase in th e density of scatterers arising from th e Si incorporation. T h e observation th a t th e two sam ples w ith th e thickest undoped A lG aN layers have th e highest m obilities and rt/ Tq ratios suggests th a t th e positive charges which com pensate th e 2D EG are rem ote from it, and possibly reside m ostly a t th e surface.

T h e versatile n a tu re of using th e SdH effect as a characterising tool has been fu rth er d em o n stra ted by using th e am plitude of th e pxx oscillations to calib rate th e electron te m p e ra tu re . From th e m easured tem p e ra tu re dependence of th e energy loss ra te in A lG a N /G aN hetero stru ctu res, th e phonon emission processes below 10 K were investi­ gated. For th e high-quality sam ple, th e power em itted followed Tg dependence, and th e com parison w ith calculated theoretical results indicated th a t th e d om inant m echa­ nism of energy relaxation by h ot electrons is acoustic phonon em ission via piezoelectric coupling w ith screening. However, around 40% discrepancy in th e m ag n itu d e betw een experim ent and th eory is observed. T his is probably caused by th e over-estim ation of th e screening effect, where, considering th e depletion of carriers in th e S chottky b arrier region form ed a t th e grain b o u n d ary interface, it can be seen t h a t for a given sam ple, th e screening effects are expected to be lower in a region w ith a sm aller g rain size. T h is explan atio n is sup p o rted by th e close agreem ent w ith th e num erical m odelling achieved in an o th er region of th e studied sam ple containing larger grains.

T h e presence of th e d irty lim it is suggested for th e low -quality sam ple, w here th e energy loss ra te followed T g d e p e n d e n c e , slightly lower th a n predicted by th eo ry for th e screened piezoelectric coupling in th e clean lim it. A lthough th e te m p e ra tu re below which th e d irty lim it occurs (T^) is estim ated to be ju s t below th e m easurem ent electron tem p e ra tu re , th e dirty lim it effect is th o u g h t to be still weakly present above Td.

7. C o n clu sio n s 162

7.2

Future work

One of th e m ain em phasis of th e future work would be on th e im provem ent of th e m aterial quality of th e grown sam ples. In addition, a wider range of sam ples needs to be studied in order to obtain a m ore accurate pictu re of th e tra n s p o rt behaviour in n itrid e m aterials. C ollaborative work w ith th e grower, w here feedback provided by electrical ch aracterisation enables a full system atic dependence on grow th conditions to be determ ined, is essential to co-ordinate th e o ptim isation process. S ettin g aside th e grow th issue, th e fu ture work regarding experim ental techniques is also significant, and th is can be classified into two categories; fu rth er analysis of th e SdH d a ta , and correlated work w ith o ther characterisation techniques.

From th e tem p eratu re- and field-dependent longitudinal conductivity in th e L an d au level tails of 2DEG stru c tu re s a t low tem p eratu res, th e charge tra n s p o rt by hopping conduction can be investigated. In SdH experim ents, th e te m p e ra tu re dependence of th e conductivity m inim a is generally pro p o rtio n al to (1 /T ) exp[—(Tq/T)^/^], w here To is th e characteristic tem p eratu re. Inform ation on th e localisation of s ta te s by th e m agnetic field and variable-range hopping betw een those sta te s can th e n be ob tain ed by analysing th e m agnetic field dependence of Tq. B reakdow n of th e integer q u a n tu m Hall effect can also be determ ined from th e SdH m easurem ents in th e extrem e q u a n tu m lim it. As shown previously in Fig. 5.17, sam ple J401 which is of a high m ate ria l quality, exh ib ited oscillation m inim a approaching zero near th e m axim um m agnetic field of 15 T . It is th u s expected th a t by increasing B further, clear dissipationless m inim a is observed in th e SdH curves. Indeed, th e m easurem ents on J401 under m agnetic fields of up to 28 T have been carried out, showing such behaviour. T he te m p e ra tu re dependence of th e w idths of these m inim a have enabled a phase diagram for breakdow n of th e Q H E to be partially m apped out, and th is work has been rep o rted by H arris et al. (2001).

O ne com m on feature of th e results presented in th is thesis is th e d e trim e n ta l effect of stru c tu re -re la ted defects, and in p articu lar, th e grain boundaries is found to play a crucial role in tra n s p o rt behaviour of b o th G aN and A lG a N /G aN layers. Therefore, linking th e cu rren t work w ith o th er characterising m ethods th a t reveal inform ation on

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th e s tru c tu ra l properties is im perative. T he proposed program m e can be briefly listed as follows:

(i) C orrelating w ith characterisation techniques such as atom ic force, transm ission electron and scanning tunnelling m icroscopy (AFM , T E M and STM , respec­ tively). As an example, the dislocation density in G aN sam ples, d eterm ined from T E M , can be used in th e scatterin g calculations carried o u t in C h a p te r 4, th u s resulting in a more com plete analysis. Also, th e form ation of grain b o u n d ­ aries and its effect as a function of th e grain size can be verified from A FM and STM investigations.

(ii) From Capacitance-V oltage m easurem ents, inform ation on th e co n centration of doping atom s and their d ep th profile is obtained. C om paring these q u an tities w ith th e values determ ined from th e H all d a ta would be beneficial in assessing th e accuracy of th e analysis. In addition, th e behaviour of co n tacts (e.g., leak­ age currents and Schottky barrier heights), knowledge which is useful in device fabrication, can be investigated by com bining w ith C urrent-V oltage experim ents. (iii) Evidence of heavy com pensation by deep acceptors in bulk G aN and do m in an t

sc atterin g by im purities introduced by in tentional Si doping in A lG aN / G aN H E M T stru ctu res, has been discussed in th is thesis. Hence, deep level tra n sie n t spectroscopy (DLTS), where studies of th e capacitance and cu rren t tra n sie n ts by th erm ally excited em ptying of electrons or holes from tra p s are perform ed, allows q u a n tita tiv e inform ation on th e presence of deep carrier defects to be obtained. (iv) U sing transm ission line m ethods (TLM ), m axim um carrier m obility and s a tu ra ­

tio n velocity as a function of applied electric field can be established in 2DEG stru c tu re s over a range of carrier densities. G roup H I nitrid es show prom ising p o ten tia l for high power, high te m p e ra tu re microwave devices, and com paring M onte-C arlo sim ulations of velocity-fleld characteristics w ith th e TL M results would provide valuable inform ation tow ards th e device developm ent research.