next , embodiments of the present invention will be described using the drawings . fig1 is a diagram representing a light - emitting device that is a semiconductor device in an embodiment of the present invention . on the back side of a gan substrate 1 in fig1 , a ti — al layer 9 that forms an n - type electrode is provided . an n - type buffer layer 2 consisting of a gan film doped with si is epitaxially grown onto the front side of the gan substrate 1 . an n - type cladding layer 3 consisting of an epitaxial film — an al 0 . 07 gan film — containing si is formed onto the n - type buffer layer 2 . an active layer 4 that is the light - emitting portion is situated between the n - type cladding layer 3 and a p - type cladding layer 5 . the active layer 4 is formed as an in 0 . 2 gan — gan multi - quantum well structure . in turn , the p - type cladding layer 5 atop the active layer 4 is formed as an al 0 . 07 gan film containing mg as a dopant . in order to secure favorable crystallinity in these layers each is formed as an epitaxial film . atop the p - type cladding layer 5 , a p - type contact layer 6 consisting of a gan film containing mg , and further , atop that , a p - type electrode 7 consisting of a ni — au metal film are provided . over that a pad electrode 8 is formed . by applying an electric potential across the p - type and n - type electrodes , a current is injected into the active layer , giving rise to recombination of electrons and holes across the conduction band and the valence band , whereby light emission is produced . in the above - described light - emitting device , the front side of the gan substrate 1 in the cleaning step undergoes a cleaning process whereby the substrate is heated to a temperature lower than the temperature of the substrate when in the film - deposition step . roughness in the front side of the gan substrate 1 is thereby held down to being 15 nm or less in root mean square ( rms ) roughness , and can moreover be brought to 5 nm or less . as described above , minimizing roughness of the semiconductor - substrate front side and forming epitaxial films onto that front side enables the crystal properties of the epitaxial films to be made superb . as a result , the quality of light - emitting characteristics can be enhanced — such as in emission - efficiency improvement and in narrowed emission - beam width . a further advantage is that enhancing the flatness of the n - type buffer layer 2 on the gan substrate to simplify the structure of the light - emitting device enables yields to be improved . peaks and valleys at a 100 μm to 150 μm pitch should not be present on the front - side surface of the n - type buffer layer ; in other words , the layer should be made not to produce hexagonal hillocks . this may be realized , as discussed above , getting the rms roughness of the semiconductor substrate to be 15 nm or less . likewise , the 10 - point mean roughness rz of the epitaxial film surface can be rendered 15 nm or less . the roughness rz can even be made 7 . 5 or less . naturally , these curbs on the roughness , through improvement in crystal properties and improvement in flatness , contribute to improving the quality of the semiconductor devices and to improving manufacturing yields . fig2 is a diagram that explanatorily illustrates a method of manufacturing semiconductor devices as described above . initially , with t 1 the substrate heating temperature in the cleaning step , the temperature t 1 is lower than the substrate heating temperature t 2 in the following film - deposition step . the flow rate of the nitrogen source gas is given as n 1 , and the flow rate of the ga source gas — trimethylgallium ( tmg ) for example — as g 1 . g 1 may be zero . hydrogen gas and other source gases may be included as well . in the film - deposition step , the substrate heating temperature is given as t 2 , the flow rate of the nitrogen source gas as n 2 , and the flow rate of the ga source gas as g 2 . in the present invention , the substrate heating temperatures t 2 & gt ; t 1 , and the condition that the flow rates of the ga source gas be g 2 & gt ; g 1 ≧ 0 is imposed . and as far as the source - material gases ( ambient ) are concerned , the flow rate of the ga source gas in the cleaning step is made lower than what it is in the film - deposition step , but the addition or subtraction of other source gases is not performed . conventionally , when heteroepitaxial films are formed using a heterosubstrate such as sapphire , removing surface contamination by pretreating the sapphire substrate in a hydrogen atmosphere has worked to the advantage of subsequent nucleation of the heteroepitaxial film . in instances in which a homoepitaxial film is formed onto a gan substrate , however , using hydrogen to pretreat the substrate promotes the breaking away of n from the surface , leading to surface devastation . the reasons for this will be explained in more detail as follows . in depositing a homoepitaxial film onto a substrate the following three events conflict with each other . namely , the three events are : ( g1 ) the breaking away of ga and n ; ( g2 ) the supplying of ga from the ga source gas ; and ( g3 ) the supplying of n from the n source gas . when a homoepitaxial film is going to be deposited , with the rates at which the foregoing events progress being appropriately related , a homoepitaxial film proceeds to form . the source - material gas supply requirements for this situation are referred to as the “ film - deposition source - gas supply requirements .” during the cleaning process ( pretreating ) gases are also supplied , to form an ambient ; for this case the term is “ pretreating - gas supply requirements .” the general practice is to have the pretreating gas supply requirements be the aforementioned film - deposition source - gas supply requirements minus the ga source gas ( group iii source gas ). under these pretreating - gas supply conditions , if the gan substrate temperature is the same as the temperature of the substrate during film deposition , or exceeds that temperature , since ga is not supplied , ga atoms left by the breaking away of n turn out to be present in excess , as does otherwise , with respect to ga droplets , nh 3 as the n source gas . as a consequence , prior to migrating into steps or scratches , relatively in excess ga ends up forming fresh gan . this results in fine corrugations being produced in the surface of the gan substrate after the pretreatment . if epitaxial deposition onto a gan substrate having such corrugations is carried out , the deposition grows three - dimensionally , and at the stage in which the gan deposition is , for example , ca . 0 . 5 μm thick , a continuous film will not form . again with heteroepitaxial film deposition , since elo ( epitaxial lateral overgrowth ) technology is often employed to create gan substrates atop heterosubstrates , the product comes out with portions coalescing in which the crystal orientations are slightly misaligned with each other . in a situation in which an epitaxial film is being grown onto such a gan substrate , because epitaxial depositions having inherited the crystal orientation of the base will at a certain stage coalesce , hexagonal hillocks will be prone to arise . the crystallinity of the epitaxial films in the completed semiconductor device will consequently be compromised , which not only degrades the quality of the device but also causes a drop in yields during manufacture . making the preheating temperature lower than the film - deposition temperature rids the gan substrate surface of imbalances from accumulation and breaking away of the atoms constituting the epitaxial film , even with a pretreating gas in which ga source gas has been omitted from the film - deposition gases being employed . a substrate front side of favorable flatness and crystallinity , suited to growing epitaxial films , can be achieved as a result . since it involves only one parameter , optimizing the substrate temperature is easy compared with optimizing the preheating - gas supply conditions . moreover , since the temperatures at which nitride semiconductor films are deposited are fundamentally high , even with the preheating temperature made lower than the film - deposition temperature , there is no harm to the cleaning effectiveness . in the foregoing pretreating - gas supply requirements , the gas flow rate in supplying the source gases that , with the source gas for ga or other group iii element being omitted , remain may be made the same as the gas flow rate during film deposition . inasmuch as the pretreating temperature is lower than the film - deposition temperature , there will be no occurrence of the imbalances noted above . therefore , simply by omitting the ga source gas from the film - deposition - gas supply requirements as described above , optimization of the pretreating - gas supply requirements by designating an immense number of parameters need not be carried out . a gan substrate was utilized , and a cleaning process ( pretreatment ) was implemented on the gan substrate , onto which a homoepitaxial film was thereafter deposited . the pretreating conditions and film - deposition conditions are as noted below . utilized as the gan substrate was a bulk crystal prepared by growing a thick gan film onto a gaas substrate with sio 2 made the mask and thereafter removing the gaas substrate . ( cf . pct gazette pub . no . wo99 / 23693 .) in the pretreatment , the ga source gas tmg was omitted , and ammonia as the nitrogen source gas , and nitrogen and hydrogen as the carrier gases alone were flowed according to the same flow parameters as those in the film - deposition conditions noted above . the root - mean - square ( rms ) roughness of the substrate after having undergone the pretreatment was assessed using atomic force microscopy ( afm ). fig3 is micrographs made using afm , showing 10 μm × 10 μm of roughness , for rms calculations , along the front side of the gan substrate . the views show results for , in fig3 a , a gan substrate with no pretreatment ( raw substrate / comparative example ); in fig3 b , a gan substrate that underwent pretreating at a 1025 ° c . substrate temperature ( present - invention example ); and in fig3 c , a gan substrate that , similarly as is conventional , underwent pretreating at a substrate temperature that was the same as the film - deposition temperature ( comparative example ). then , rms calculations for the foregoing samples within 2 μm × 2 μm and 10 μm × 10 μm bounds are set forth in the table . according to fig3 and the table , the sample pretreated at a substrate temperature that was the same 1150 ° c . that is the film - deposition temperature had an rms roughness of over 20 nm ; thus its surface roughness compared to that of the raw substrate on which pretreatment had not been carried out had grown drastically , deteriorating the substrate quality . the size of a single gradation along the z - axis in fig3 c is ten times that of the z - axes in fig3 a and 3b , and although the magnitude of the disparity just noted is thus somewhat hard to discern , the disparity is unmistakable . on the other hand , with the sample ( present - invention example ) pretreated at a substrate temperature lower than the film - deposition temperature , the 2 μm × 2 μm rms roughness is 0 . 6 nm , which is markedly superior to the 1 . 3 nm of the raw substrate . the 10 μm × 10 μm rms roughness , 1 . 5 nm , is superior to the 2 . 0 nm of the raw substrate , and proved to measure one order of magnitude smaller than the 23 . 6 nm rms roughness of the comparative example . as described in the foregoing , by pretreating a gan substrate at a substrate temperature lower than the 1150 ° c . film - deposition temperature , the substrate is cleaned without its flatness being harmed . as described in the foregoing , by forming an epitaxial film onto a gan substrate that has been pretreated at a substrate temperature of 1025 ° c ., an atomically stepped surface in which the rms roughness is 0 . 5 nm or less can be achieved . fig4 is views showing results of using differential interference contrast microscopy to observe , following stages , the surface of a sample in an example of the present invention . fig4 a is a photograph in which a gan substrate after being pretreated correspondingly to the substrate in fig3 b is observed by differential interference microscopy ; fig4 b is a photograph of a gan film at the point at which it has been epitaxially grown , according to the foregoing film - deposition conditions , 0 . 5 μm thick onto the gan substrate ; and fig4 c is a photograph of the gan film after it has been epitaxially grown 2 μm thick . in turn , fig5 is views showing results of likewise using differential interference contrast microscopy to observe , following stages , the surface of a sample in a comparative example corresponding to that of fig3 c . fig5 a is a photograph in which a gan substrate after being pretreated correspondingly to the substrate in fig3 c is observed by differential interference microscopy ; fig5 b is a photograph of a gan film at the point at which it has been epitaxially grown , according to the foregoing film - deposition conditions , 0 . 5 μm thick onto the gan substrate ; and fig5 c is a photograph of the gan film after it has been epitaxially grown 2 μm thick . as will be understood by comparing fig4 and 5 , the disparity in surface roughness of the gan substrates after the pretreatments is clearly confirmed by differential interference microscopy as well , with the roughness in the surface , shown in fig4 a , of the present - invention example being less than that of the surface , shown in fig5 a , of the comparative example . furthermore , when a comparison of the gan films at the point at which they had been deposited to 0 . 5 μm on the post - pretreated gan substrates is made , from fig4 b of the present - invention example the gan film proves to have been formed continuously , while from fig5 b of the comparative example , the gan film turns out to have grown three - dimensionally without the layer forming continuously . likewise , from a comparison of the gan films at the point at which they had been deposited to 2 μm on the post - pretreated gan substrates , it is evident that from fig4 c of the present - invention example a flat gan film formed , while from fig5 c of the comparative example , hexagonal hillocks were generated , in which the just - noted discontinuous film that grew three - dimensionally came out coalesced . in other words , in the comparative example a gan film of inferior flatness and crystallinity formed . fig6 and 7 are diagrams showing the results of measuring , using a stylus surface profilometer , the surface roughness of gan films deposited at 2 μm thickness , corresponding to those in fig4 c and 5c . in the fig7 roughness profile of the comparative example peaks and valleys whose height / depth is 50 nm to 150 nm , at a pitch of 100 μm to 150 μm are ascertainable . this is analogous to the roughness from the hexagonal hillocks verified in the fig5 c differential - interference - microscopy photograph . in fig6 of the present - invention example , on account of the film - deposition process having been carried out after pretreatment at 1025 ° c . was performed , no heavy roughness can be ascertained . addenda regarding embodying mode and embodiment example of the present invention in the embodying mode and embodiment example , only an example of a gan substrate and a gan film formed atop the substrate were described , but the scope of the present invention most broadly is not restricted to gan - based semiconductor elements — they may be other nitride - based semiconductor devices as well . the amplitude of the roughness of the semiconductor substrate , etc . in the semiconductor devices of the present invention is based on the roughness prior to a thin film being formed on the substrate , etc .— with the description , including that of the mode for carrying out the invention , presuming that even once other thin films have been formed on the substrate , etc . the roughness will not undergo a significant change . nevertheless , the actual amplitude of the roughness in the front side after it has been fabricated into semiconductor devices will depend significantly on the measuring method — particularly in implementations in which the surface roughness is exposed by etching , on the etching technique . and it will also depend on the precision of the device with which the roughness is measured . it is believed that , in determining the roughness amplitude of the surface of the respective areas in the semiconductor devices of the present invention , the best measuring method and the best measuring device have been specified . in the foregoing , an explanation has been made of modes for embodying the present invention , but the embodiments of the present invention disclose above are ultimately illustrative examples ; the scope of the present invention is not limited to these embodiments . the scope of the present invention is indicated by the recitations of the scope of the patent claims , and includes meanings equivalent to the recitations of the scope of the patent claims and all modifications within the scope . utilizing according to the present invention nitride - based semiconductor devices and a method of their manufacture makes it possible to achieve nitride - based semiconductor devices containing epitaxial films excelling in flatness and crystallinity .